Summit Racing sells CCA-4758-2 Competition Cams Low Tension Valvetrain Checking Springs for $3.69 and with some good hardening modeling clay that will consolidate in a few minutes so you wont have much error when you peel it off or separate it from the piston top. With a playdough, the situation is tricky so you're not pretty sure. To me, I dont think a .001" will make any difference when the recommended is .080" and .100" on the exhaust/intake valves. You're looking at a marginal sufficient clearance here.
Note that when you grind or flycut the piston top with an Isky piston notching tool, you will alter the piston weight so the best thing is to use a digital precision scale to keep all the pistons of the same weight or you will have a balancing problem.
For better instructions, read this article
http://www.fordmuscle.com/fundamentals/pistontovalve/