Question: Where do you expect to find a 7 1/4 LSD? And if you do it will probably need new clutches, which are pretty much made of "unobtainium". The last set of clutches I bought (after looking for a few years) cost over $150.
PS: I did run a 3.91 SG 7 1/4 in several 13 sec drag cars with slicks, and still have one with 5.13 gears in it.