The only thing you need to do to a 340 is more carburetion or gears. Why are they Legendary? Because they were the best all around engine ever produced, as is. The "68" 340 4 speed would go into the 12's. Put a Thermo-quad carb and intake and they will really fly. NHRA had the "71" factored up in the 325hp range or even higher, and that was before they allowed you to port the heads. With the right gears you could get 20 mpg or beat a 365hp 327 4 speed Nova by 6 car lengths. And that was a daily Driver, what more do you want to do? Doesn't get any more low buck than that.
