Given a bit of training, Usain could very well be a total beast at 500m. Those fast-twitch muscle fibers, and he's clearly freakishly adapted to move his long legs quickly. And those guys train damn hard with more endurance stuff than one might expect.
As a big, big boxing fan, I think about some of the bigger, more athletic heavyweights (a rarity these days!).
Anthony Joshua is a big dude, about 6' 6.5" (199cm), is very strong, and he's surprisingly athletic and moves his body and his feet well. If he doesn't break the machine, he'll produce a very good 500m.
Wladimir Klitschko at about 6' 5.5" (196cm) is also a very good athlete who's worked very, very hard on his overall strength in the last ten years. Wladimir also has a very long reach, so he'd be getting a couple of bonus inches at the catch, and like most dedicated boxers he's actually very fit. It wouldn't surprise me if he could already go sub-6:00, as I've seen him using the erg in training and he's a very clever man so he's probably learned how to use it properly. He's a 250-pound machine.
30, 6'2 (1.88m); 179 lb (81 kg)
Learning, improving, getting stronger, and wanting more.

Recent tests: 1:41.7/500 for 1k; 1:34.9/500 for 2 minutes