Well you see, Tank Abbott was built like a tank, but he was also an abbott, it's just that wildcard factor of any great fighter. Beer was like spinach to him and the only times he lost was when he was forced to be sober by Dana and his cohorts.
Now Brock, well, his name speaks for itself, B-rock. He's built like a rock, B-rock. I mean as much as Tank was a tank, could he destroy a rock? I dunno. Not even the Hulk could really beat the Thing fully so that should give us some kind of idea of what would happen.
In the end, I give a 91% chance to B-rock and 84% to Tank, so pretty even. But yeah, you try punching solid rock and tell me you can break that with your fist. Impossible.