In today's era they'd have all the access to training, nutrition, sports science and recovery. And fight videos. They'd be fine.
Joe Louis would struggle. Chins would hit his hands with such ferocity that he would see many an early night. No chance he can 30/40 punches thrown a round last his way to see the final bell most of the time.
Any heavyweight that is good enough to be called a heavyweight great would find some sort of success in any era.
I mean it's a boring answer but it's going to be guys like Langford and Fitzsimmons who wouldn't be fighting at heavyweight in 2026.
The real early guys were like 5'7 sometimes so Lanky Bob and the like but idk if that's entirely fair because they're so far out of the modern concept of hw. Some of the champs during the late 40s and early 50s were light hw even in their times so guys like Rocky and Charles would struggle
