stock 340 is really about 9.5:1, I assume he's going for around 9:1 not much difference.
engine size really has nothing to do with hp. just where the powerband is and torque.
If you build a 318 to 340 specs, should make about the same hp at about 300 rpm higher.
Valve shrouding might steal a few ponies but at 275-325 hp probably not.
Idk 318 with 9:1 340 heads 4 bbl headers and possibly more cam depends who he listen to.
sounds like a 325 hp engine to me.