I'd go with a 650 DP on a 340. I am running a 650 BG claw on my 340 4 spd Duster with the MP .474 cam and 3.91's, works awesome, just rips the hides off!!
Just buy a proform center for your double pumper. They flow about 830 cfm after the change and they work great. I have that set-up on my 408 in my Duster. Make sure you change your jets to compensate.