Moorer took some monster shots from Foreman without going down, until the final roof of the mouth splitting finale. Don't know if Spink's chin is as tested at heavyweight.
In all fairness I think Spinks would give Foreman a competitive fight for a few rounds, and land some good shots before eventually going down. Like Lobotomy said, the man could box and an upset is not out of the question, but I would certainly have Jinx as a big underdog.
Sometimes it's not all about power, though. Holyfield, Moorer and Morrison all had respectable power to say the least, but neither of them damaged Foreman the way Stewart did, over 10 rounds. Boxing is complicated.
Have those tipping Old George to win ever seen him fight? He was far too slow to beat spinks. Spinks would hit him 3 or 4 times before he landed one. Foreman in his comeback was upfront about ducking the boxers and movers. He only wanted to fight the fighters that came straight at him, because they are the one he could land on. That is why he wanted to fight Tyson. Most opponents knew this also. Holyfield, Morrison, Schulz, Briggs and any other fighters that troubled George all miraculously discovered how to hit and run when they fought old George. Those that didnt were beaten. As soon as Moorer compromised this running/moving style he was KOd. Spinks would not need to trade or want to. This fight would be very similar to the Holyfield Foreman fight. Although i always thought that Spinks hit a little harder than Holyfield and was generally a little better. I think Spinks has an excellent chance to stop old George in the later rounds. With the exception of old Holyfield, he was a class or two or maybe even 3 above anyone old George fought. Obviously a Prime George Foreman is a different kettle of fish.