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If I understand the question right, the answer is that both V8 and I6 use the same cross member and bell housing for stock manual transmissions, and the cross member location does not change ( I have no idea how an automatic mounts as have never owned one ). V8 and all but 292 I6 use same motor mount pedestals in same locations. 292 uses a unique pedestal mounted further forward on just the pass side.
A good way to tell if things have been messed with is if they are bolted to frame, as all I have seen are riveted if original.
This info applies only to factory original, hope it helps.
Good luck, Tom