what is largest cam for 340 with factory 10.5 flattops????????

You really have to check it for yourself because machining tolerances were all over the board. I have had heads that wouldn't allow more than .450 lift and others that will allow slightly over .500 lift. That's the lift at the valve taking into consideration the lift loss from the poor geometry and less than 1.5 ratio rocker arms inherent with small blocks meaning you will never get a full .484 lift at the valve with a .484 lift cam. It's usually on the order of .020-.025" less than the cam is rated.

I have to disagree with the comment that more duration = more power and low end torque and more lift = more RPM. It's just the opposite. Duration affects the rpm range in which the engine operates so the more duration the less low end torque as a general rule.