I know this is an older post but I have owned two 75 Grand Prix model LJ. Each was equipped with a 400 4BBL and a 2.56 open differential. The first one I had in the mid 80s had an 8 1/2” 10 bolt cover and carrier with 9 1/2” drums. If memory serves me correct it was built late in the model year. I presently have one that is an early build, I think November 74 and it has the 8 7/8” 10 bolt cover 12 bolt carrier and has 11” drums. This is what my 75 Pontiac service manual says they all should have. I have no reason to suspect the rear in the one I had years back was ever replaced. It seems during these years they put in whatever was available. This must be a known fact as when I bought a set of rear brake shoes they asked if they were 9 1/2” or 11”. I’m guessing an easy way to know what style of rear axle is what size the brake drums are.
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