Finding a '73-6 A-body 7 1/4 rear end with the big bolt pattern shouldn't be all that hard to find.....finding a good one might be though. It would at least be a bolt-in without having to change the driveshaft or anything else back there except obviously for the wheels. And no, you can't just swap the later axle shafts into your '66 7 1/4. They use different axle bearings. I'd just bite the bullet and get a heavier duty rear with the bolt pattern and gear ratio you want and not mess with a 7 1/4. What sort of horsepower are you going to put through it?