He would have to be a strong favorite over both. They would be good stylistic turds to throw at him though!
Both trouble him immensely. Lightheavies caused him more trouble than the scant few true heavies he faced. Machen was light years ahead of stumblebums like Brennan who troubled Dempsey. I imagine the best version of Jack eeks out a decision but Machen went the distance with Liston, Patterson and Williams so durability under heavy fire was more the rule than exception.
Stumblebum is harsh for Brennan. Machen was never an easy nights work though and was very skilled and tough. If Jack can keep it on the inside he can win. If Machen fights from range and Jack can’t close on him could be Eddie’s night