Its all too interesting..And could never be settled unless in someway shape or form it happened..
Which we know is obviously impossible...
Roy outspeeds. outskills, and has the potential to make a rocky marciano look foolish..
But roy could not knock him out..
But Rocky was in great shape..A total mauler.. fit enough to keep coming for 15 rounds and a very heavy hitter .Especially for an 180 pounder..
Say this was at lightheavy..Roy has some power there.. Still would say there would be no chance of a roy jones ko though..
Also one factor is that in both the jersey joe and the ezzard charles fights rocky came back from knockdowns to win by knockout.. So who knows ..It might have come down to the length of the fight or timescale in which it happened.. example..

Rjj- marciano in the 50's 15 rounds.. I might be inclined to think an extremely late stoppage for rocky..
Or over such a period rocky might be stopped on a factor like cuts..

The same fight over 12 rounds in the last twenty to fifteen years i would say that rocky would be stopped by a referee most likely..

Peace