Who runs a six pack 340?

750 Holley vacuum secondary carbs have seemed to work well with mild 340s.

Also, Carter T-Quads can be very good, as these have small primaries, and huge secondaries.

Also, have you thought about the Holley 670 Avenger? A buddy of mine has one on his 360 Dart Sport and it works well, out the box, with a Comp Cams XE 268, headers, RPM Air Gap and Mopar ignition.