lift really has nothing to do with P/V clearences. When peak lift if achieved, the piston is at the bottom of its stroke, P/V clearences come into play when the exhaust gases and leaving the and intake is coming in and both valves are open when teh piston is at its highes point. It has more to do with valve size and duration than lift. I know 2.02 valves usually need modifications to stock 302 pistons. Smaller is usually okay, but anything larger than a 2.02" will need a flycut piston i beleive.