If Joe Louis fought in the late 80' to 90' against those heavy weight champs

A crappy, out of shape, awkward slow fighter is still crappy out of shape and awkward wether they're short, average, or tall. A modern guy who has these qualities going in a time machine is not suddenly going to become an all time great in another era. Sure the extra couple inches of height might help them bully lower level opposition but if they never made it beyond C level in their own era it's not like they're going to just flip the script. To suggest otherwise implies size is everything.

Even if we assume boxing technique has "advanced", that only means that if you took the old school crude oafish fighters and raised them in today's era with the same modern methods, I don't see why they wouldn't be able to have similar or somewhat close amount of success as their modern counterparts.