i have not done much racing, but i have an idea on setting up a race engine. if running an engine well above factory top rpm, should not the bearing clearances be much bigger than stock?

 

The thickness of the bearring doesn't matter much (unless you are getting way in depth, and then it probably involves something like weight over gain in performance... but dont quote that). What matters most is the quality of the parts. For a high revving engine, (say 7000 to 10000) EVERYTHING needs to be built to run that fast; Heads, pushrods, crank, cam, valves, springs, lifters, special pistons, down to the block.