I think it's been mentioned several times here that the Hi-Po's got the 9". As to the other part, good question, one I've wondered about. Lots of lists showing which cars could be had with either size but never seen one that links what engine to what rear. Would certainly think choice would be just based on motor torque I think that the only 9" rears that came with the 31 spline were the 427 and 428's, again - never seen that in good print.
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