Re: how effective would the lighter heavyweights be today?
I don't think all of the guys you listed have the same kind of problems. The two most successful ones are Evander Holyfield and David Haye, both 6'3", 215 lbs, with 78" reach: Ali's measurements. They might have fought low for awhile like Ali did at the beginning, but 215 is their natural optimal weight. They aren't like the 6 foot tall guys who started at middleweight and moved up to cruiser.
Some of the modern guys you listed aren't much smaller. Adamek and Marco Huck are nearly 6'2". So their natural weight is probably around 205, which would still be fine if they fought in the 60s or even the 70s. I'd like to consider them separately since Adamek actually has a decent heavyweight record whereas Huck has had one fight at that weight and lost. Adamek doesn't have the same punching power to KO the big guys that he had at his natural weight. Above 200 pounds his record changes from 70 percent knockouts to 70 percent decisions. He can still get wins, but not against the elite and not in impressive fashion.
The 6'1" guys like Dempsey are probably right on the line at 195-200 pounds if they don't cut weight. Dempsey is a special case because he could really punch. He knocked out an old Jess Willard which shows he has comparable power to a 220 pound guy like Luis Angel Firpo who accomplished the same feat at around the same time. But there's no evidence that he knocks out A level 6'6" guys with good chins in their prime. I kind of see him doing like Frazier and Tyson with the hard punch, speed, bob and weave, style; cutting a fearsome swath through the division for a few years, burning out early, and getting beaten by the top A listers. The accumulation of punishment from that in fighting style would wear him down fast and if he doesn't retire early he'll be getting tuned up by everybody like Mike. He's actually a lot like Mike Tyson in that his knockouts come in the early rounds and if you could get into the 5th you are mostly safe. We have evidence of his KO power against Willard and Firpo, but he finished both in about 3 rounds and we don't know how well he could handle boxers that size over the distance.