X heads on a 340 HP potential???

A stock engine with headers will dyno 325 or better.

If your useing headers ..... I'd bet 400 HP at the flywheel.
I get this quick number from the oldMP create engine, the Comando 360 pumping out 360 HP with stock 308 heads & 2.02's (Approx. 200+ cfm) and a Purple 284/484 cam. Between the porting, larger cam and the better HP making shorter stroke of the 340, you'll be at 400 or better HP.

The create engine also use's the MP M1 single plane and 1-3/4 tube sized headers. The newer RPM (Relitive) should produce as much HP as the single M1 at this stage of the game.

The newer Magnum create engines use the .509 cam and above mentioned parts and make over 400+ on the dyno, no matter who dyno'd it.

Considering a (?!) 25% loss through a 727 trans .... you'll get 300 HP at the rear wheels. So, depending on what he is thinking ...........