Alot of the alphabet champions then were athletic skillful boxers there wasn't really a heavy handed slugger or a real KO artist like Foreman for example. Maybe Bonecrusher could be dangerous to Frazier early with his physical presence and power. You also have a contender like Cooney who is a very big guy aswell who has KO power another dangerous opponent for Frazier in the early going. I would say someone like Thomas would be a problem aswell because he's extremely durable the issue is he hasn't got a varied offence he relies alot on the jab and can't fight on the inside. All in all stylistically I think Frazier would match up well vs most of the 80s Heavyweights apart from Tyson.
Yes not many guys that were seen as big hitters really back then. Smith and weaver come to mind but unless they get extremely lucky early, then Joe gets em. Maybe smith survives the distance but I see Joe taking out weaver. Guys such an Page,Tubbs and Tucker might win a few rounds but they ain't beating Joe. Cooney is dangerous but once again he has to be extremely lucky to beat the smoke.
Tyson, Ruddock or Bonecrusher,but all of them would've been dragged to hell and back by Frazier.. If they win,I think it would definitely be a close SD