You might want to check out my engine build thread here:
427 side oiler engine build "Blog"
I used clay and did a "test" assemble to check for valve to piston clearance issues (427 vs 429 but the principle is the same). During this procedure I ALSO discovered my push rods were to short. It's just good "buiseness" to do a "dry assemble" on your motor and check ALL clearance issues.
.050 below deck piston height and a "nominal" lift in the 500's should leave plenty of clearance I would think. Check out the "Squish and Quench" section of my build regarding piston deck height issues and WHY I like "zero deck piston height" whenever possible.
Edit: With the clay, after you've hand rotated the engine a few cranks, just cut the clay in half on the top of the piston. Measure the depth of the clay from the valve impression (if any) to the top of the piston. Should be at least or around .100 thousand.