Replace the valve springs on one cylinder with light "checking" springs. You can get these at most any hardware store. Just hold the valves closed but not enough to collapse the lifters. Then use a dial indicator on the retainer and hand turn the engine to/through 20° BTDC to TDC and then on the other valve from TDC to 20° ATDC. This will tell what valve clearance you have now with you lift. The way to measure is every 5°, open the valve by operating the rocker arm until the retainer touches the stem seal or the valve touches the piston. This will inform how much valve to piston clearance you have and at what ° closest approach is. Depending on your pistons, 0.508" lift these days is not excessive. Older engines only used 0.350" to 0.400" as the spring technology was not as developed. Now you can run 0.600" on the street reliably, provided you double check all clearances and use recommended springs.