It depends on what frame of mind "the Atomic bull" entered the ring in. For the sake of argument, he is in good shape and up for a tussle. I think George would plod forward to a UD, with no knock downs. I predict a fairly average unexciting match due to styles with George's jab keeping McCall on the backfoot.
McCall by decision. Comeback Foreman was slow and easy to hit. McCall could easily take the punches he did get hit with.
McCall by boring decision. The " killing machine " Foreman was easily exposed by Ali, and Young, the lean mean grilling machine version carefully avoided any real threats, or simply lost to better classed ones.
Now this would be a war if both are in their prime! Jeez, how didn't I think of this fight? Great fantasy matchmaking, OP! ;-)
All those guys you mentioned who beat McCall were faster than Old George. Even Bruno. The Mike Hunter fight you can throw out because it was very early in McCall's career and Hunter was extremely quick and unorthidox. Tucker and Douglas were boxer/punchers. I think both of them could have beaten George as well. No shame losing close fights to Douglas and Tucker. Norris was fast and had an excellent defense. He was a very good contender for a short while. I agree both George and Oliver were not particularly quick, but McCall was a little quicker than Old George and extremely durable. I think it would be a close fight but McCall would slightly outhustle George for a close decision win.
Smoke and mirror comebacking Foreman with the bribing help of Arum would never consider fighting McCall..........wayyyyyyyyy too much risk with no reward. They carefully handpicked glass or at best mediocre chinned fighters......Morrison, Stewart, Saverese, Schultz, ****ey , Briggs and Moorer were never known for there granite whiskers.......still some of these" Hall of famers" beat him or should have gotten the decision while turning his face into pizza. McCall was a solid contender who waxed guys like Lemmie, Oleg and Akinwande.......better than what Foreman did in his comeback. prime vs prime Foreman has to get him out early, after that McCall is a live opponent. I met McCall several times and saw him spar live with Tyson, he is a big guy and strong as a ox. Bruno had him seriously hurt in the late rounds but did not have the gas to finish him, that is the closest of anyone to stop the Atomic Bull.