Hughes Edelbrock heads : Smallblock

flow of 300 cfm is a lofty goal. Bigger is NOT always better. 300 cfm head on a 340 would = High rpm with NO low rpm power. If that's what you're after and are willing to modify your hole drivetrain to fit.......Then you're on the right track!
I'm guessing that you're not.
Listen to MRL and let him guide you into the right package, for your application.
YOu still haven't told us what that is?