Punching technique yes (bigger gloves has meant the post 50s fighters never had to worry about that), but Tyson (at his best) had the fast feet, Dempsey never had.
Tunney has excellent mobility against whom? Against old "I forgot how to cut off the ring" version of Dempsey? Against grossly overrated smaller Carpentier? Like I said, he didn't get his mobility working when he faced not too old great skilled fighters, neither was his jab working effectively. All three names I listed showed better boxing skills than Tunney himself in their meetings.
Lennox Lewis Awful style for Tunney, I think Tunney could survive if he fought passively, given Lewis' lack of a killer instinct, but I feel he'd just be overpowered taking the fight to a much bigger superheavyweight. I'd have to go for a midrounds Lewis stoppage. Mike Tyson At face value, this is the perfect style for Tunney to fight against, a comeforward swarmer in the Dempsey mould. I feel that while Tunney might have some success for a few rounds, Tyson would just be too much for him physically. Tyson was more explosive than the version of Dempsey Tunney fought, stronger, and a little bit better at countering those straight shots. I'd go for Tyson midrounds TKO. Jim Jeffries This would be a war, and a darn good one. Tunney had tremendous heart and courage, and he'd need it in this fight. Jeffries was tough and I could see him stealing a few rounds, but I think Tunney would take the UD with his better pure boxing. Sonny Liston Conceivably this is the fight Tunney may be able to win - Liston's handspeed was not that great, and he was heavy on his feet, but scientifically he was better than all the aforementioned fighters. His timing was superb, and once he landed a big shot he finished you. Tunney would have to be very precise and careful to take the decision from Liston, I'd give him a 50/50 chance with the other 50 being a Liston stoppage at any point of the bout. George Foreman It took a mighty good boxer to do it, but I believe a prime Foreman could be boxed despite his ability to cut off the ring and bombard his opponents. His punches were never that precise or quick, and Tunney, the masterful pure boxer with a fantastic 1-2, might be able to stop him very late. I think Foreman would be favourite, but I'd favour Tunney 60/40.