John B (vegadad) has provided information about identifying a rear end here.

To get a good guess, you can jack up the rear of the car (BOTH wheels). Mark one tire and mark the driveshaft. Turn the driveshaft until that tire makes one revolution. While you're doing this, count the revolutions of the driveshaft. This number of revolutions is your ratio. Your ratio is not likely a whole number so you'll have to estimate how much of a revolution the driveshaft makes as the tire finishes its rotation. The number of revolutions is the ratio.

While you're in there, you can upgrade to a new or different posi carrier by getting a 26-spline carrier of the right series for your gears (see question 3.10).

NOTE: Installing new gears requires setting the proper pinion depth and carrier position. Doing this wrong will destroy your gears. Even the most die-hard shadetree mechanics leave this to a professional. If you want to try it anyway, this is the basic procedure and tips from Robert (twelve_second_vega):
"When you pull the carrier, mark the shims on the side of each bearing. They are cast iron and must be handled with care. Put a new crush sleeve on the pinion and install it in your 7.5" housing. Set the preload (about 10 inch pounds with used bearings) and set the carrier in place. Gently tap the shims you took out into place with a PLASTIC hammer and torque the caps. Only 1 gearset I installed needed to be reset. 99% of the time, you can just throw them in and go. If the backlash is off (.005" - .008" with used gears) you will need different shims. Subtract the amount you need (in backlash reduction) from the right shim to move the ring gear deeper into the pinion. BE SURE to add this SAME AMOUNT to the opposite shim. You MUST maintain the preload on the side bearings. When properly set-up, you will have to pry the carrier out of the housing and putting shims back in should not be easy either."

Bob (botizan) adds this tip:
"When installing F-body or other 7.5" gears in your H-body car, you can keep the H-body U-joint by using the yoke from a 76-79 X-body car (Nova,Apollo,Omega,Ventura)"

Yes, but you'll need to transfer (cut then weld) the brackets for the coil spring suspension since the S-10 uses leaf springs. Also, you'll need to figure out how to mount the torque arm if you have one, or revert to the previous 4-link rear suspension.

Lee Abel of Abel Performance (Phantom402) gives more details:
"Found out a neat thing today the springs on an S-10 are exactly the same width as the rear control arm mounts for my 75 2+2 factory v8, so I'm gonna cut down the springs, clamp the leafs together, and bolt it in. Add the spring mounts and panhard bar and its done except the brake line... no problem, tack weld piece of metal to frame some where convienent and hook up s-10 hose. BTW some later 80's earlier 90's S-10's came with 8.5 10 bolts so all ya gotta do is up grade to a 2wd vesion an boom ya got a rear end that is about as strong as a 12 bolt after a lock right, moser axles, "c" clip eliminators, summit pre load cover, and mobil 1 gear oil. If ya find a f-body from 70's or early 80's you can swap the disc brakes."

Yes, but first you'll need one from an '82 or newer car with the torque arm suspension. Then you'll need to have it narrowed if you're not using the wide IMSA body panels. Make sure the brackets end up in the right place! Finally you'll need to modify the torque arm mounting area.

Roger (monza_madness) gives more details: "The stock F-body (3rd gen 1982-1992) rearend will fit an IMSA panel Monza using 10" wide wheels with 4" back spacing. No shortening required, just relocating the H-body spring perches and H-body control arm mounts. Widen the H-body control arm to fit the differential housing. The front differential mounting holes have to be redrilled in the control arm. The control arm upper and lower ridges have to be notched, and control arm has to be pulled to a 90 degree angle to the rearend and the ridges have to be rewelded. Careful not to use the German made firebird rearend, identified by finned aluminum brake calipers, as this is a light duty rearend. Also can be identified by the 9 bolt differential cover.

"When moving the spring perches and control arm mounts, I used the differential case as a reference point. Then I double checked the distances between the spring perches to guarantee proper positioning. I used an angle gauge mounted to the pinion bolt holes, set at 5 degrees down, then used the level to set the perches and control arms. For the pan hard bar mount, I installed the rearend with spring perches and control arms tacked in place into the car (monza). With control arms and springs and torque arm in place, I lifted the full weight of the car with the rearend. The mount, with the pan hard bar attached, was placed on the rearend centered with the marks that were measured off the old rearend. Check that the wheels are centered in the body, then tack the pan hard mount into place. Then I removed the rearend and finished welding everything into place."

If you're hooking up well (for example, using slicks and getting low 60' times in the 1/4 mile), people seem to agree on 300 rwhp. After that, start looking at aftermarket 12 bolt or 9" or similar rears. Don't get an S-10 rear thinking it will be stronger because it probably won't be. Also, people with Camaros making just over this much power break rear ends, so don't look too hard in that direction either. If you're making much more than 300 rwhp, you deserve a custom rear anyway. :) Don Hardy 12-bolts seem to be popular.

If you're not hooking up, then it almost doesn't matter how much HP you drive through the rear end because you're just spinning the tires anyway!

Some have reported that a stock Chevy 10 1/2" flywheel won't fit. Hayes 30lb 10 1/2" will fit: part #490-10-330, 153 tooth. A flywheel from an '88-up Camaro (153 tooth) will also work. Note: use diaphragm pressure plate if using a stiffer clutch to prevent stress on clutch cable/firewall.

Bob (venom_symbiot) writes:
"I installed a larger flywheel and clutch in mine and tossed the recessed Monza unit. Everything fit inside the factory V8 bell housing. I would suggest you take the bell housing and see if it will fit over the new flywheel and clutch assembly. If it does, your next concern will be the starter motor. If it clears the bell housing then the only thing that could possibly be a problem is whether it clears the uni-body or not. If it does contact the uni-body, it is usually a simple matter of grinding or cutting the offending material away (as mentioned in Hooker Headers installation instructions). I would add that once the material is removed, if any spot welds were also removed that you get a MIG welder and run a bead along the area to fuse the layers of the uni-body back together to retain structural integrity.

It's not as hard as it sounds, I didn't have to remove any material to clear my starter, the flywheel fit, as did the starter without any modifications. Eventually, I replaced the bell housing with a Lakewood scatter shield that I modified to accept the Monza clutch fork and clutch cable."

It's a tailshaft counter balancer. It dampens vibrations in the drivetrain.

This is very subjective, of course, depending on what you want to do with the car being the biggest factor.

For a street-driven car, Robert (twelve_second_vega) recommends:
"I favor the Saginaw due to its wide variety of available ratios. I would like to share with you the combination I am using on my Vega, not that it is amazing, but rather an example of compromise. My Chevrolet 302 is a 9.8:1 Compression engine using Stock Iron "993" heads and the GM "Pop-Up" piston. The intake is an Edelbrock #2101 fitted with a Q-Jet that has 76 jets/.051" Metering Rods, "BH" Secondary Rods and a "B" Hangar. The Hydraulic Camshaft is a mild 467" 490" - 225 236 @ .050" with a 113 L/C. A basic, straightforward engine.

I am using Monza Motor Mounts and Manifolds including the factory 1975 V8 Monza "Y" Pipe with (1) 2-1/4" pipe exiting in the stock location through a single Dyno-Max "S" Turbo Type Muffler. My concern was having an agressive ratio down low, yet still retain good cruise speed and economy (relative) for extended driving. As a result, I discovered the following combination.

Using a 2.92 Axle ratio and the 3.75 First Gear ratio of the 4-Cylinder Saginaw 4-Speed. The Final Drive Ratio (FDR) is 10.95. Lower than the 10.41 FDR of a 2.54 First Gear (T-10) and a 4.11 Axle Ratio.

Performance figures look like this:

First Gear @ 5,000 RPM = 35 MPH
Second Gear @ 5,000 RPM = 60 MPH
Third Gear @ 5,000 RPM = 95 MPH
Fourth Gear @ 5,000 RPM = 130 MPH

After each upshift, RPM returns to near the center of this engine's powerband:

RPM Drop @ 1-2 Upshift = 3,000 RPM
RPM Drop @ 2-3 Upshift = 3,200 RPM
RPM Drop @ 3-4 Upshift = 3,600 RPM

While not ideal for racing, I hope this example may prove helpful to those that are searching for a useable combination, for a street driven car, with parts that are laying around the garage."

See question 7.10.

And some more from Clyde (CJBIAGI):
"I agree with Kevin, you need to stick with a diaphram type pressure plate. I used a complete Hayes setup, 30lb steel flywheel and 10 1/2" street clutch, this is the same clutch assembly that would be used on a heavier Camaro or Chevelle and is more than adequate for a light car like a Monza. If you did want to step up to something heavier you may consider the Centerforce Dual Friction clutch. This is still a diaphram clutch but has much higher holding force and a light pedal feel which should not strain the stock cable or firewall. BTW, the correct way to adjust the clutch is with the adjustable pivot on the top of the passenger side of the bellhousing. Adjusting the cable is NOT the way to get the correct pedal freeplay or desired gap between the clutch disc and flywheel. Hope this helps."