Bolt pattern adapter?

Just use the rear out of a 73 up (although some 4 wheel drum brake cars still used the small bolt pattern that year so watch out.) If you swap to a 7 1/4 then it should go in with no mods at all, brake lines, driveshaft, U joints, U bolts, everything should be the same. If you go to a 8 1/4 you will need to shorten the driveshaft or use the one out of the donor car. A used 7 1/4 rear end with brakes should be well under $100 (or even free) and I bet there are lots of them in junkyards that don't crush their stuff.