Performer RPM will fit fine, or at least it does on the 64/5 cars. An E cam would be kind of a waste on stock P heads, but itll give you the lope youre after. With some porting the P heads flow decent and will save you a bunch of money over the aluminum heads. Either trans will fit, but the AOD may take a couple whacks of the 5lb install kit, or it may not. I know its tight, but certainly doable. The only rear end that will bolt in to the round body cars is the Maverick 8". In about 72 they upgraded the center section with cross ribs and slightly stronger axles. Even if you do get aluminum heads and a bigger cam, I wouldnt worry about breaking the rear end, especially with an automatic.

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-Eric
65 Comet wagon, 60% of the time it works every time.