What are you doing for top end ? If your using just the 340 top end (heads) you may gain a few hp, but what your mainly doing is lowering the powerband in rpm, eg.. Say your 340 engine likes 3000-6000 rpm a 416 might be 2500-5500 rpm or less.
I plan on porting my J heads (or just invest in aluminum heads and get those ported), hydraulic roller, and some bolt ons. Going to a machine shop today to gauge how much they would charge here in NY.