Calzaghe wins on points, the sport had evolved lots by then and their balance is far better and they know more in terms of they know more ways to set up punches nowadays
I think Tunney is pretty bad for Calzhage. Hopkins used his positioning and counters to lower Cal's output, I think Tunney would do similar but also use body punching to do it a bit better.
Tunney would quickly figure out that Calzaghe was busier than him, and that trying to outbox him on the outside was a bad idea. He would then switch to the front foot, and go for a body attack, which would quickly turn the fight in his favor. Calzaghe had never been stopped, so I will venture Tunney on points.
By the descriptions from his day, Greb sounded like a smaller Calzaghe, a quick, shifty, high-output guy who used tons of angles. Tunney's game was pretty basic, linear, not terribly imaginative... a poor man's Henry Maske, if you will. Nice on film but mostly fighting scrubs. Calzaghe by convincing decision.
Tunney takes this,,,I like Joe but I see no one on his resume that makes me think he beats Gene Tunney.
I was 50-50 on this matchup, tbh. Tunney is really a good boxer on the outside. Then Seamus reminded me of the parallels (probably) between Greb, who gave Tunney problems, and Calzaghe. That tilts my opinion towards a Calzaghe decision. These are both top-of-the-line guys in their eras. Not that easy to evaluate.
I always got the impression Greb was a much rougher fighter than Calzaghe, and not as 'consistent' (if that is the right word, maybe 'set'?) in his Boxing compared to Joe, who was a stand out amateur. I do agree with you that this is a tough fight for both, and I definitely see Calzaghe confusing Tunney in the beginning. I don't think he'd get stopped by Tunney, but that is a bit of a what if IMO. I can see it as being one of the hard to score fights where Tunney might have some more effective blows, especially towards the end, but Calzaghe would try swamping over him and land at a higher rate, and going around him with his feet. I can see this, in a best of 3 scenario, being a match up where Calzaghe wins the 1st bout but Tunney adjusts and wins the other 2.
Tunney is actually a good body puncher. He's got more pop too. I think Calzhage chin becomes suspect at 175 and higher. Tunney would out punch him and like you said lower his punch output.