Any historical boxers you think this modern style of weight training would have bennefited?

Sure that could've benefitted them but doing weight training as the main form of training will result in accumulation of useless muscle mass and will slow the boxer down. Joshua suffered this same fate against Andy Ruiz in their first fight.

George Foreman & Lennox Lewis would've become absolute beasts in powerlifting given their natural strength and size.

 

Not really. The boxers who adapted strength-based style (Carnera, Foreman) were already naturally gifted when it comes to physical strength and could further accentuate it with exercises more benefitting a boxer than weight lifting.

 

Yes, assuming that they also have whatever nutritional support Joshua did. At least in the general sense that modern training would probably improve whatever athletic qualities old-timey training tried to develop.

 

I am pretty sure guys lifted weights back in the 70's, 80's, 90's. Also lifting weight doesn't make you a great boxer. If anything if you get to jacked up it will make you slow and effect your stamina. Joshua lifts weights and do you honestly think that makes him a great fighter?

Also George Foreman past his prime clearly lifted weights as you see doing his comeback.