Scratchee, you use the feeler gage just like when adjusting the valves between the tappet and the top (bottom on a flathead) of the valve stem. The clearance should be around .012 when the valve seats so using a thicker gage just give a little extra kick on the opening stroke. NOTE, the only time there is contact between the valve and tappet is when the cam is pushing it open and the valve spring is pushing it back down following the cam to the seat, once on the seat the tappet will drop another .012" giving your valve clearance. That is how a solid lifter tappet works.
There is a ramp angle ground into the cam that uses that .012" gap to "GRADUALLY" contact the tappet instead of just slamming into it. There is far more to a "simple" cam profile that most would think.
When a valve sticks it sticks while traveling back down following the cam profile just as it does on the way open, this is the "valve timing". As the cam pushes the tappet the tappet pushes the valve open against the spring pressure. Once the valve reaches fully open the "spring'' pushes the valve back to it's "seat" and holds it tight against the cam profile. So if the engine turns with stuck valves they are sticking on the top of the stroke or on the way back down. Because if they stick on the way open the engine stops turning or something breaks!!!
Adding a bit more "lift" by using a thicker gage just gives the valve a lil extra push usually breaking it free at the top. Once you have some motion of the valve the oil can get worked in and the valve should follow the tappet following the cam profile. It does not take much movement to get the motion started again and the spring will push it a closed a little more each revolution. Just don't add a bunch more spacing as you don't want to "bottom out" the spring coils.
I strongly suggest using the starter motor with the spark plugs removed and fully charged batteries. This spins the engine fast work the oil pump to keep the oil moving through the engine properly. Turning by hand will not keep the valve tappets oiled against the cam lobes.
Dave Neptune