piston to valve clearance 1955 F100 Y-block

i am trying to push down the valves on a 1955 ford F100 Y-block to tell if they are rusted stuck. most of the valves will move but a few of them don't move at all. how much valve to piston clearance is there at TDC. could i just be pushing the valve into the piston?

 

If some won't move at all they are most likely stuck, but you can rotate the crank half a turn and then try again. They can't be at tdc in both places. There is enouge clearance at tdc to move them a significant distance, anyway.