if your running a solid roller lift cam with good spring pressures and an automatic tranny I've got away with .060 exhaust valve to pistons and .050 intake valve clearances in a bracket motor that got raced a lot with no issues other than not much exhaust soot under the valves position on the piston tops
You can always cut the valve reliefs in the pistons in the motor with the proper tools, Isky use to offer them, not sure if they still do or not