How legit is the heavyweight title before Johnson?

Not relay, because these fights couldn't have been made anywhere close to Sullivan's prime.

 

Sullivan was essentially retired when Jackson came on the scene. He had suffered a badly broken arm, at least one alcohol related coma and was paralyzed and given up for dead with typhus (or just as likely alcohol-induced sickness)...

 

I won't speak on all of them.. But boxing was far less organized when Sullivan was champion. The heavyweight title was a new concept after all, and guidelines which determined who was the best challenger around weren't entirely clear. He could have faced Peter Jackson and perhaps a few others, but again, the sport was in its embryonic phase.

 

If we are talking about fighters who were active anywhere close to Sullivan's prime, I would say that McCaffrey was the best gloved challenger available, and Killrain was the best bare knuckle challenger available.

 

Sullivan is deeply in question from 1886 on ... Corbett was able to demand terms and then fought an injured Jackson , never giving him a rematch. We have no clue on Jeffries as he refused to fight Johnson, a huge mistake on his part that got him big later. It goes on and on.

 

I really don't think either of these matter that much.

Even if we assume the worst case scenarios, their respective title reigns would have been shortened by about a year.