Pro wrestling and boxing were inextricably linked around the time of Jeffries and Johnson for a few reasons. There were promoters (Jack Curley being the obvious and most famous example) who would do shows in both fields, and also a lot of the theatrical tours that were popular at the time would have wrestlers and boxers both on board; I know Jeff toured with a travelling troupe of boxers and wrestlers pre-Reno. I guess friendships were forged along the way and that's one of the reasons he had Martin 'Farmer' Burns as one of his main trainers for the Johnson fight, plus, depending on who you listen to, William Muldoon as well.
Muldoon, of course, did give an in-ring speech prior to the Jeff/Johnson fight about fair play, etc., aimed at calming the crowd down. But I've heard contradictory things about whether he was a bonafide part of the Jeff camp or not.
This was around the time pro wrestling was becoming a bit of a mystery to the public in terms of how 'real' it was, but I don't think there's much doubt that wrestlers were all legit tough guys back then and did command fairly universal respect. Many were seen as pioneers of conditioning and physical fitness which again must have appealed to boxers, particularly to someone like Jeff who had such an incredible amount of weight to lose in the buildup to Reno. Burns and Muldoon both had terrific and long-lasting reputations as far as personal fitness and resistance training goes. Off the top of my head I know Gene Tunney regarded Muldoon as a mentor and credited him hugely for helping his career, I'm sure there's a fair few other examples too.
In terms of tactics for the Johnson fight, it isn't hard to imagine Jeff was hoping to outmuscle him in the clinches and stay clear of Jack's uppercuts, so training in wrestling would be a logical move. Didn't work out too well though...
Originally Posted by Johnstown
I know Johnson wrestled in exhibitions, and those seemed to have been "real" wrestling matches.
I am 99% certain that any wrestling match Jack did will have been worked/faked, would be absolutely amazed if not. Some pro wrestling matches had predetermined endings as early as the 1880s (there's evidence to suggest the majority did), and it's super hard to believe Jack would agree or even be asked to do a shoot/real bout when it'd be easy, hassle-free money to have a standard worked match.