I ran a 650 Speed Demon DP on my 340 about four years ago. It was a nice carb but I wanted something with more adjustablility and an electric choke.

For the last three years, I now run the Quick Fuel Super Street 750 Annular DP (#SS-750-AN: 735cfm is actual rating per QF). It comes with the electric choke, adjustable secondary linkage, replaceable: air bleeds, idle feed restricters, and pvcr's. These adjustable features sure helped me dial it in when using an O2 sensor for tuning.
It is a very nice running carb and I would definately recommend it on a mild built 340. Like Drag said, it is very responsive on the street.

Note:
QF makes allot of their own carb parts and they are usually cheaper than Summit (Air Bleeds: $5.26/pair, IFR's: $5.52, Main Jets: $4.89, High Flow PV (QF standard equipment): $6.32)

Note2:
When I called QF with my car combo, I asked for the SS-650-AN and they told me that since the annular booster takes away 15cfm, they suggested that I try the SS-750-AN instead. The tech thought that the 650 would be too small for my setup.