Bob, I don't have an answer regarding the use of adapters. Most will tell you not to. My suggestion is to find a pair of axles from a 73 and later A body 7 1/4 rear end and install them.
OP, i had the same size tires on a stock 68 dart, 4 wheel drum and 7.25. Now i had the front low and would rub until i removed that brace from the top wheel lip, then i was golden. that said i was just using rally wheels with no size or BS for you but i bet you need more BS with that spacer