My son has a 68 Cutlass with the 12 bolt cover, non-posi 273 gears. He wants posi with 355 or 373 gears...which they don't have much for these rear ends I've noticed. Whats the best choice, get a 68-72 Chevelle 12 bolt rearend, it should bolt right in, what about driveshaft length as he just had a $450 driveshaft made for his car.

 

12 bolt chev differential will require drive line to be shortened to replace the Olds 12 bolt. I would look for a 71-72 8.5 10 bolt from a Cutlass. It will have the bolt in axles and uses corporate 8.5 10 bolt carrier and gears. I think you can use the same driveshaft you have with the 10 bolt not 100% sure.

 

If you keep the O-Type, you are limited to 3.42, 3.90, and 4.10 for new ring and pinion gears. For a posi diff, either used GM O-Type (671 or 588 casting numbers) or new machined Ford 8.8 carriers are the only things I am aware of.

Swapping to a 71-72 corporate 8.5 inch 10 bolt opens up the range of gear ratios and posi choices.

 

2.73 gears were never offered. It is a 2.78.


You can get carriers & appropriate gears from several vendors who are board members. Rebuilding a rear end is not cheap. You have to pay to play.

 

I found a tight GTO 3.55 posi on CL for $500 locally. Not a big deal to install. Bolts right in. Takes all the same brake components.

 

Are you talking about an 8.2" 10 bolt? If so you paid a fair price. They are not strong and don't like sticky tires.

If that is a 12 bolt Chevy 3.55 from a 71-72 455 HO car then you stole it.

 

Agreed. My brother-in-law has the original 3.55 cone style posi behind his GTO and it has held up for years behind a stout engine, but he also has tires with almost no traction. It's one of those differentials with a split case and 10 bolts holding the halves together so it has some inherent weaknesses.

 

if it's an auto even the 8.2 should hold up fairly well