340 Piston to valve clearance w/RHS heads

Best thing to do is assemble the short block and check EACH piston and measure how much it sticks out. You may find some that are too tall requiring a different gasket. You won't know what you need for sure until you do this. It only takes one to ruin the party!!

That and don't forget those old trw pistons fit loose in the bore and will rock while the engine is warming up. The clearance will decreases when the motor is warming up.

Hyper pistons, while weaker won't rock much and can run tighter clearances.