offenhouser or a airgap?

im building a 340 for my 1970 340 duster and i belive it is 10:1 comp. it has light wieght pushrods. I want to make 400hp and putting j heads w/ 2.02 intake. not sure what carb or cam to use. i dont care if my idle is ruff. I have the offenhouser intake.