Put an LSD into the 7.25 with 3.55s,,shim the diff parts as required to reduce the backlash in the spiders,add some good oil, and call it done.
You probably don't want to run slicks on it very often tho.
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.