[Bill Harris]

Actually, SS has it's own piston rule section and it is pretty liberal. You can do pretty much anything you want as long as the piston top is the same configuration as OEM and it doesn't do anything to increase the compression ratio. In most cases, the compression ratio will be less since the valve reliefs will have to be deeper than OEM due to the high lift cam. Stock has to have the OEM ring pack and placement as well as the OEM top configuration and volume. I think most all-out SS engines use very light pistons with only one narrow or high tech style compression ring and a narrow low-drag oil ring. They might look like the OEM from the top of the motor, but you would never mistake one for OEM if it were sitting on the bench.
That being said, an OEM piston should be perfectly legal, just not as trick as it could be.