falcon and mustang are basically same chassis.
as in newer, do you mean 1970's? 80's?

things to consider: overall wdith, so teh wheels fit in teh wheelwells (with $$ this can be adjusted)

suspensions style (coils, leafs, etc) - kinda gotta be the same.

then you have different styles/strenghts of rear end...8", 9", etc.

So as Streets asks, what is the purpose ff the change? If your old one is just bad/worn/broken, are you just looking for a cheap fix? or are you looking for some type of performance improvement?