Qawi at 5'7" weighed 232 for his 1998 career finale, the same Foreman weighed for him in 1988. On an inch for inch basis, that might at least make Dwight the heaviest competitor to be enshrined in the IBHOF.
Lionel Butler was 5'10 or 5'11 and actually exceeded 280 in some of his later fights. As for "under 280", he managed a 279 for a fight in 2003. James Toney weighed 257 at 5'10 in one of his fights.
Not fat, we can start with Carnera, who was 270 when he defended against Loughran. His height was 78 inches. That gives him a reading of 3.46 lbs per inch Tyson was only 3.15 lbs per inch, at 221, assuming a height of 70 inches or 5'10.