On an engine like this I would forget the clay. Degree the cam where you want it, install light checking springs on both valves in cylinder number one, install head on block with a spare gasket of known compressed thickness or leave gasket out completely, and install lifters, pushrods, and rockers. With degree wheel showing true TDC, turn engine to 10 degrees BTDC and measure clearance with a dial indicator by pushing the valve open further until it touches piston or retainer hits guide. Repeat this at 10 degrees ATDC. Been using this method for years and I always check it to 20 degrees both ways just be sure and never had an engine with less clearance away from the 10 degree positions. This was from the Reher-Morrison engine book. I forget if intake or exhaust is closer at BTDC or ATDC so I am not specifying, just check both since it's easy. Also, double check every cylinder this way, especially since you know you have a heavily decked block.
