The match up favors LaMotta as he was durable, and Hearns was best at welter. I would however give Hearns a chance to win via decision if he fought a smart fight
For one fight at 10-12 rounds, Hearns by decision. A 15 round title fight, LaMotta by late round stoppage.
The idea of Hearns "boxing smart" and decisioning a strong aggressive middleweight is purely theoretical at best, and pure fantasy more likely. It crops up often, some people even profess to still believe that Hearns could have outboxed Hagler. In this regard Hearns is overrated. Even the bulked up Hearns had trouble keeping strong aggressive middleweights off him, and any time he was presented with the challenge he was outbrawled, KO'd or forced to trade and KO the opponent in a vicious or wild fight.
LaMotta by late stoppage in a 15 rounder (not 12)...and would have to be a prime motivated version of LaMotta to beat Hearns, the one who is determined to 'walk through hell with a smile' as the song goes... Tommy Bell outboxed LaMotta, and didn't get the call...Hearns is more than capable in this...Very slight edge to LaMotta in my mind.