Kaz weighed way more than 280...in his prime he was around 340lbs at 6-3.
id say 315 is certainly believeable for him for a few cheat curls. lee priest says hes done that for a few reps one time. Kaz was maybe the all round strongest man ever, weighed 340 lbs, longtime lifter/juicer, id say he was definitely capable of doing 315.