Do you know aprox. how much milage/usage is on your rebuild? From what I've been reading the RHS heads from IMM Engines are going to be hard to beat for the money, and you can't go wrong with a good old MP purple shaft. I'd go with the 280/474, that cam runs awsome in a 340. Top it off with a good dual plane intake, RPM or RPM air gap. And you should have a stout little 340. Before you buy any heads, I'd pull your old ones and look at your pistons. If you have a flat top with valve releifs your OK, but if you have a factory style or TRW 2316 that stick out of the deck, they can only be used with factory open chamber type heads!