Well, not all wagons came with an 8inch. Mine didn't.
To the op.
It is a must that you find out what rear end you have before making any plans. If it is in fact an 8, you have lots of choices. If it is the 6.75, you pretty much are sol.
Some Mustang IIs came with the 8 inch that is a bolt in swap. Early Mustangs (64 to 70) can be used, but need a little modification to the leaf spring perch. And there are others. Anything from a fox body up would need spring perches added.
Russ