If it was purely down to butting heads, you'd have to back Gene whose head was a bigger threat than either of his hands. Zale was the better all round Fighter by a distance but even he might have struggled to put the rock hard Fullmer away. It would be a great fans' fight though.
I would take Zale by a split decision in a fierce fight. He hit harder and likely backs Fullmer up and keeps him on the defensive while still muscling his way inside.
For what it's worth, I believe the Zale pre the war, would have been too fresh and forceful for Fullmer, we have nether seen Zale at his peak, when he was trading with the likes of, Conn, Abrams, Hostak, Mamakos, and the very tough and durable Apostoli, whom he floored three times on route to a wide points win, plus the left hook to the body took many a MWs legs away over the long haul, perhaps a more interesting scrap would have been with Tiger !! stay safe buddy.
Blimey Mike ! You do pick 'em. Zale and Tiger is an absolute war ! I'm going to have to think about that one. All the best mate.