Joyce. He will be able to use his height and jab to box Foreman from a distance. Both guys are slow but Joyce is a bit quicker and his reach advantage would allow him to win a UD. Joyce is too smart to brawl with him. Both guys will land some heavy punches but ultimately Joyce will win a decision. I actually favour the younger Foreman to tko Joyce but that is a different story all together.
Parker's and George's chins and punching power are in two different realms (whiskers closer than paws though), but Joyce is too smart to try to juggernaut at Foreman. He will UD with his jab and engine.
Joyce beats 90's Foreman, too busy for an old Foreman and I don't think Foreman can put in the kind of sustained attack needed to KO Joyce. Now 70's Foreman that's a very different question.