7 1/4 to early b-body (1962-63) 8 3/4 swap?

Those years used the tapered axle shafts.
Replacements to standard flange shafts are available, though. Just be aware that they will also require later brake drum setups (drum, backing plate, hardware) in addition to the replacement axles.
An accurate 8 3/4" rear axle width list