Heavyweights don't tend to work their way through different weight classes facing different top men from different divisions, no. Some HW's do sometimes fight men that are heavier. I think a fighter who was coming in at around 210 and dominating this HW era, for example, would recieve additional p4p recognition.
Still a 6'7'' fighter weighing 240 lbs being undisputed heavyweight champion and retire with a record of 54-0 with 52 by KO, having 20 title defenses against top competition could never be p4p #1?
No. I wouldn't say "could never be". He would need a pretty astonishing resume to compete with Langford beating greats from Gans through to Wills though, from 140 to HW.
He's my pick for p4p #1, yes. I'd say Robinson, Greb and Armstrong are also reasonable picks. I've never seen a list from a member I really respect that had Ali at #1, although he has crept into my top 10.