The place to measure the piston to valve clearance will be at the top of the exhaust stroke. Watch the exhaust valve spring as you rotate the engine in the normal running direction, and as it relaxes fully, the piston will be very close to the top of the cylinder. Then you can either probe into the spark plug hole, or use some other method of assuring the piston is at top dead center of the exhaust stroke like using a borescope, or watching the ball and pointer in the bellhousing window.
Once you're sure the piston is at the top of its travel, use a long, heavy screwdriver or a pry bar to push the intake valve down against the top of the piston. Stock stovebolt cams have very little or no "overlap" where both valves are open- - - - -the exhaust valve is almost fully closed before the intake begins to open. More radical cams, like the one you have, will probably start opening the intake a bit sooner- - - -maybe even a LOT sooner. The exhaust valve is angled so it simply cannot touch the piston, so you only have to be concerned with intake valve to piston clearance. As long as there's at least 1/16" or more of free movement before the valve touches the piston at the top of the EXHAUST stroke, you're good to go!
Jerry
