Swap out 7 1/4 RE for an 8 1/4 RE

Before trying to answer your question, I think I'd have to ask "why are you doing it?" If the 7 failed and this is just a straight repair/replace, then why worry about it, just use the same parts?

OTOH, if you are building towards some higher performance and don't mind cutting/welding perches, then you have already contradicted yourself, because it stops being simple at that point. In this case, I would try for an 8 3/4. The A-body units are pretty rare nowadays, but you should be able to get a C-body narrowed 2.5" each side for $175-200 and have a very stout setup. Granted, even the C-bodies are getting hard to find in the boneyards these days.

I guess my point is the 8 1/4 option is a compromise in both ways. Probably gonna require some modification anyway (driveshaft, whatever), AND not the best in terms of strength either.