Monzon, after a slow start, finds the range with the jab and spears the onrushing Fullmer. Gene's rough-house tactics don't work as Monzon starts firing tthe right with accuracy by yhe middle rounds and busts up Genes face. With both eyes just about closed by round 12, Gene's bull rushes are timed by Monzon and the right gets in with sickening precision and power...finally decking Gene at the end of the 14th..but the bell saves Gene. In the last round Gene mounts a desperate rally, but he's practically blind and a series of rights deck him again, and the fight is stopped near the end of the 15th as Gene is badly staggered. A bit of controversy as the fight is stopped so near the end of the fight, but the ref's concern is for the almost blind and battered Fullmer as the methodical Monzon can't seem to miss and was teeing off at will and with power.
Fullmer would be ideal for Monzon, who was stronger than Robinson, and utilized his reach and height to better advbantage...ever so patiently piling up the points with the jab and firing the right in a methodical manner. There would have been nothing but frustration for Gene, as nithing he did would have work..he would have had little success muscling Carlos around.
I definitely don't envision any kind of stoppage in Monzon's favor, especially if he's having such trouble with the likes of Bennie Briscoe in his prime and Rodrigo Valdez later on. He'd use his height and reach to keep Fullmer at bay for the most part, but you bet your ass Fullmer takes it to Monzon from start to finish. He'd probably look to fight off the backfoot at times, luring Monzon in as the aggressor behind his jab, only to pounce the way he did with guys like Lausse who put him on the retreat. An inside rough-housing contest between Fullmer and Monzon would've gotten ugly, but in the end Monzon's superior outside game wins the day for him.
Fullmer might make it interesting (and ugly) for a few rounds as Monzon warms up, but then Monzon begins to rip Fullmer with inside combinations and discourage him from getting inside. Once that happens, he basically controls the fight to a UD.
If it happened in the western united states fullmer would win. That guy got so many gift decisions and gift draws out there it was criminal. You could have shot him in the head with a bazooka and they still would have found a way to make him come out on top. That being said anywhere else and monzon wins it.