Easiest way is to file down the casing mounts on the 7.5" diff as mentioned so it fits in your cradle and swap the output shafts around from the 7" into the 7.5" diff -that way you dont have to change the existing shafts or wheel bearing flanges.

Its easy to do, just remove the diff case and push off the c clips holding them in place and pop them out and then fit the others, refit the c clips and refit the cover.

I have a mint 27,000 mile from new 7.5" diff here spare that came out of a bog standard show 3dr that was still on standard chip, so never ragged or abused, just nicely run in if interested.