I have a 72 cutlass 442 conv 455 engine and a th400 tranny with a 323 rear in it now.It originally had a 350 with a th350 tranny in it with a 323 rear.The engine has a little work done a better cam bored out i thing 20 or 30 over has 14 inch SSII wheels.Will i blow the rear end or should i up grade to a 12 bolt opinion's please

 

Your car should have the 8.5 rear. Under no circumstance do you swap a 8.5 for the O axle 12 bolt rear. The 8.5 rear axle is equal to or superior to a 12 bolt Chevy rear which is superior to the O axle 12 bolt. I think for highway driving the 3.23 behind a BBO is about all the gear you want. If you do have the original 3.23 8.5 rear and decide you want a lower gear like a 3.73,you will have to change the carrier. Parts for the 8.5 are plentiful and reasonably priced. I hope this helps you.

 

How do i know if its a 7.5 or a 8.5 rear

 

There wasn't a 7.5 in a 72 Cutlass,only 8.5's. Starting in 71,all Olds A-Body cars had the 8.5 corporate rear axle. 70 was the last year for the 12 bolt O axle in A-Body cars. If I'm not mistaken,78 was the first year for a 7.5 in the Cutlass. The O axle 12 bolt rear axles only has 10 bolts holding the ring gear to the carrier unlike the Chevy which has 12. The only reason I would use a 12 bolt O axle would be for originality.