You should be worried about PTV clearance on the exhaust stroke, when the exhaust valve is all the way open, and the piston is at the top of its rotation. Compression stroke at top dead center shouldn't be a problem because the gas has already been sucked in and both valves are closed in preparation for the ignition of the spark plug, AKA the power stroke.
This is where the clay comes in, (or you can use a tootsie roll and tape it to the piston) With the heads off, tape the tootsie roll with the wrapper on or stick the clay on the piston where it might encounter both valves. replace the gasket and heads temporarily, without tourquing anything, and slowly and carefully turn the crank a couple turns, if it gets tough, or you feel like you are going to break something, (like a valve) back the crank up and pulls the heads. If there is any part of the piston visible through the clay, or if you cut the tootsie roll in half then you will need to do some notching. I don't know the exact tolerances for clearance but if it's pretty tight you may need ask a engine shop. Good Luck