Rear end upgrade??

Tube diameter is the easiest way to tell 7.25 from others. The 73 with LBP front disk brakes could very well have LBP 7.25 rear. I've got 2 of those laying here, one from a 73 V8 build and other from 75 \6 build. These are plentiful and cheap.
Reading you comments suggests to me that you will loose your drivers license long before you go through two 7.25 rears.