Ali and Frazier were very different fighters. Ali was great at avoiding a punch, Frazier needed to take one to deliver one, something that doesn't work against Foreman. Schulz and Moorer are not that different, smaller HW's, good boxers, little power at HW. Besides, Ali had every conceivable advantage in Zaire, that win was historic and can hardly serve as an example. Moorer had nothing to exploit Wlad's chin problems. Nobody beats Wlad with boxing skills, especially not a 6'2 fighter who is not lightening fast. Moorer is the type of fighter that Wlad feasts on. Arreola would have a better puncher's chance to beat Wlad than Moorer.
I think he'd do very well, like a lot of good boxers from that time... I would put my money on him against Haye anytime, but against one of the brothers, don't think so.
Shannon Briggs once said when asked who hit harder between Lewis and Foreman "No offense to Mr. Lewis, but when Foreman hit me, I saw Jesus!"
He probably would have been a top 5 guy. Unlikely he could do much against a Klitschko, and I think he'd have pretty decent problems with Valuev, Povetkin, Solis, Haye and possibly Arreola (although Haye-Moorer would have been a hell of a fight).
But it's also obvious that the comeback version of George wasn't a knockout artist against elite opposition anymore. He failed to knock out Briggs or Morrison, who didn't have the best chins, or Schulz who would have been knocked out 10 out of 10 times against Wlad.