Bennie could be a little passive sometimes, and I could see Hearns winning the fight with the jab. However, I would go with Briscoe wearing Hearns down and stopping him, what round I can't say.
Though Hearns didn't have the toughness of Monzon, but his jab was much quicker, and his right hand power was equal to Monzon, perhaps better. Briscoe would sometimes become hesitant l when a good jab was used against him. Hearns's jab , height and reach advantages, and the threat of that nuclear straight right hand would keep Briscoe thinking when he should be attacking. Hearns in a 12 rd fight would win the fight easily. a 15 rd fight would be very tough for Hearns but he should have enough in the tank to hold on to win a closer dec.
Monzón didn't have the skills to completely shut Bennie down, and even he was hurt by him. I see Hearns using his speed and length to pin point right hands on Bennie's chin, but primarily trying to box until getting caught and finished. Something like a prolonged version of the Barkley fight.
The Hearns of the Hagler fight would have destroyed any fighter that stood in front of him, the hand speed he showe in the first round of that bout landing several straight rights that would have knocked out any other middle was eletrifying, too much power for Benny, however Tommy lost his way after that bout, if he came out lie he did in the Shuler fight he would have stopped Benny also, after that it`s 50/50.
Thomas Hearns with all his skills sometimes did fade, maybe due to stamina issues as seen against Iran Barkley. When you meet a prime pressure fighter like Bad Bennie Briscoe, who lasted 25 rounds with Carlos Monzon, in two bouts, even staggering him in the 9th round and going 34 rounds against Rodrigo Valdes in three bouts, you have to be on top of your game. I think that the fight against Bennie would have the flavor of the Barkley fight, Briscoe could take a punch, he was stopped by Valdes in 1974. Bennie was a tough Philly fighter. Bennie stops Hearns in round 12, after taking his best and wearing him down.
The vastly stronger Bad Bennie overwhelms, overpowers and stops Hearns by 7. He takes his share of punishment from Tommy, but proves to be too relentless and steam rolls Hearns.