No, Haystacks Calhoun was a huge guy who carried a significant amount of bodyfat but other than sheer size he doesn’t really bring anything to the table to defeat Brock Lesnar in a shoot fight.
Calhoun was billed as being 6’4” and 601lbs. He wasn’t as immobile as many guys his size are, but he there’s no way he would be giving Brock a run for his money in the speed or agility area. Calhoun would in effect present a standing target for Brock.
Brock should be able to outstrike Calhoun by using his superior speed. Brock’s striking skills may not match up to those of the likes of Alistair Overeem but against the plodding Haystacks I think he’d have little trouble landing power punches at will.
Could Brock take down a man of Calhoun’s size? I would think that the answer is a resounding yes. Brock Lesnar was a tremendous amateur wrestler who was blessed with freakish strength and explosive speed. I think that, should he decide that it was the best course of action, Brock would have little trouble double legging Haystacks.
There are many stories of Brock manhandling professional wrestling’s big men, including the 7’ tall and over 500lbs Big Show. Kurt Angle looked back with awe at witnessing Brock throwing Big Show around during an impromptu shoot wrestling match before a WWE show. Kurt told Justin Barrasso of Sports Illustrated in 2015;
“Brock was in the ring with Big Show. He was literally throwing this 530-pound man around the ring like a little kid. I’d never seen anyone lift up Big Show like that without his help, but Brock was picking him up and throwing him around.” (Kurt Angle on his failed return to WWE, wrestling Daniel Bryan)
If Brock can dominate Big Show in such a manner then I think he’d have no trouble doing the same to Haystacks Calhoun. Prime vs prime, my money would be on Lesnar in a real life combat encounter.