Mike: I believe what you describe is piston-to-cylinder head clearance. Actual deck height is the distance the top of the flat surface of the piston is from the top of the
cylinder-block deck with the piston at top dead center. The minimum for this on a 427 is 0.0005 inch, but 0.018 inch will surely work. Whatever deck height is, you've got to ensure there will be no piston contact with the cylinder head.
Check out how to determine deck height by clicking on:
http://www.2strokeheads.com/Deck_Height.html