I think the most common was 2.56. According to the 1975 Pontiac service manual the rear was supposed to be the MP 8 7/8”. This is the rear with the scalloped 10 bolt cover similar to the 8.2” but has a 12 bolt carrier. I have owned two 75 GPs. Both had 2.56 open. 1 was a late year build and had the 8 1/2” 10 bolt with 9 1/2” drums. My present one was an early build originality titled in November of 74 and it has the MP 8 7/8” rear with 11” drums. It has been upgraded to a 3.23 Safe T Track. Both LJ models with the standard 400 4bbl.
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