340 tooooo hot

Well since you had it rebuilt my guess is you had it bored and now that you have thinner cylinder walls all that heat is building up in your coolant and would cause the temps to be high...............just joking but I had to throw it out there.

My guess would be improper tune...too much/little advance or running lean. Both will get your motor to run very hot fast. Also I imagine you have more power then before which equals more heat but I've never seen it make a drastic change in temps on a street car. How much timing are you running and when is it all in by? Also how do your plugs look?