SmartKen: To answer your original question, IMO, the engine is over-carbureted for street use. Besides better mpg, the smaller carb will give improved throttle response at lower rpm. For race use, the 770 is still a bit much for the 340 assuming 6600 max. rpm), but it's better than going down to a 650 and running out of breath at top rpms.
Your low end stumble may be a result of premature opening of the secondary. It's probably worse when the engine is less than completely warmed up. Let's see what others say about putting in a heavier diaphragm spring.