Here's a great article on 340 HP numbers. I have a 69 340 X head, just getting ready to break the cam in. Factory iron heads, steel rockers, steel crank. I currently don't have specs on internals- see story in "340 break in question"
After reading this article, I understood that getting to 400 HP was much more attainable than I imagined. HR dynoed a stock 340 with headers and a factory 6 pack and I believe an Edelbrock intake at 363 HP.Choose your headers carefully, I recommend Doug's for overall fit. They're 3x the cost of cheaper headers but worth it, they don't hang low and the only mod I needed was at P/S box, 1 small dimple. How Bad Did Mopar’s 340 beat Ford & Chevy? Shocking New Data!