I had a 66 Mustang with the 6 cyl in it, and it was 4 lug, but had the 7.5 rear in it. My brother has a Falcon with an 8" rear, but it's 5 lug, small circle. I know some MII owners talk about using the early Mustang 8" axles for a budget 5 lug conversion, so I think Dmoody is right.
If you need to stick with 4 lug, why not just redrill them with the proper pattern?