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!!

Yep, I would not try to run close chamber heads on a 340 with pistons sticking out .018. I have the same thing but milled .030 open chamber iron heads and I went with the .055 fel- pro's to play it safe.

Any chance the head can be machined open?