Vega
My '76 Standard Vega HB had the 6.5, 2.92 geared rear end in and held up just fine with a built 4.3 & TH350 set-up. The S-10 rear can be used, just need to cut off the leaf spring pads and weld on the 4 link brackets.
If you get a rear from a 4 cyl S-10 with manual trans, it will have either 3.73 or 4.10 gears (1st gen S trucks). The v-6 trucks used either 3.08 or 3.42 depending on options (A/C).
Also, posi was standard in S-10 Blazers (check first), but the rear is a bit narrower than the truck model. I also found that the 8.5 rear came in some of the 4x4 trucks, but more work is needed to make work (6 lug axles for sure). The guy to speak with is Monzaz on the V8Monza.com website. He is the rear end guru on H-bodies.
If you keep everything in the stock location, the driveshaft will work with the TH2004r trans. But since you have a wagon, and high horsepower, get a good shaft made.
Another thing to remember is that the radiator for a V8 Monza will NOT fit a Vega. ($295 dollar mistake on my part), instead, use the HD V6 w/air cond. radiator. Plenty thick and will do the job. You also need to add a route for hot air to escape from under the hood. If not the motor always heats up and could kill it. Long trips require popping the hood (rear facing, no probs) to let this build up of heat escape. I'd imagine that adding a turbo will make this heat worse.
Be sure to check over the frame extra good (ask how I know) near the cowl. Even in Cali the car was known to rust out there. I know of no way to fix that area if it is rusted. An S-10 front stub will physically fit into the space, but then attaching it is another problem. Fixed mine by going tube chassis, but then stock parts won't work.
Good luck with the project & post some more pic's!
Jim
'69 GS Califorina - 4 speed
'76 Vega - tube framed
'82 Turbo Regal Sport Coupe
'64 Nova 400 series - current project, 454 powered