First of all this would be a very ugly fight. Both are very defensive boxers. Ali has the edge in footwork and chin and Roy has the edge in handspeed. Who wins? I think that Ali will fight a way to win. EDIT: the fight is p4p
I assume this means at HWY and not some mythical P4P matchup. I don't see Roy having any realistic chance.
p4p doesn't favour heavyweights, and Ali is no exception Ali would get hit with the left hook all night long
At HW it's Ali by far. lb4lb doesn't really work for HWs as they tend to get penalised way too much for their greater size and strength.