I think that Brock's cardio is good for his size, but insufficient for a fight against Randy. If we look at his last fight, where he spent the vast majority of it riding Herring, Lesnar's pace slowed dramatically as the fight carried on. If Herring had the tools to actually put up a fight, I'm very sure that Lesnar would have looked worse than sluggish in the 3rd.
Randy's clinch is what I believe will push Brock past his cardiovascular threshold.
+ the clinch is a power struggle Lesnar's strength/size and Amature Champion level wrestling put him at a large advantage....
Wrong. Clearly you don't train or know anything about the clinch. If you have nothing but strength, then EVERYTHING is a power struggle. If you possess sound technique, other factors besides raw strength come into play -- such as leverage and timing.
As Hex Rei said, Freestyle Wrestling (which Brock did) is more about shooting than clinch work. Greco Roman, on the other hand, is all clinch. Unless you've actually worked with somebody who did it, I doubt that you could comprehend the intricacies of that sport. And I hopefully don't have to tell you that there's a difference between shooting and clinchwork.
Even more so, having re-watched the fights, Brock's shooting isn't even that good from a technical standpoint. Two of his 3 shots had no penetration. One of them got nowhere; Heath just turned around into a failed attempt at a Judo throw. The other one had no penetration either. Brock just powered through it. He put his shoulder against Heath's midsection (but not hard enough to topple his balance) then pulled on his legs and fell into it.
Sure, it worked, but that won't suffice against Randy.
Unless he's gotten significantly worse over his year off, Randy will have the advantage in the clinch and probably score a takedown. That is, unless his head gets taken off by a XXL sized hand.
(Either way, I will sig-bet with somebody on the outcome of that fight. )