Choosing the right carb for my car (340 4-speed)

You hit the nail on the head there buddy! All soooooo to often, the thinking pattern is to run with the larger carb on top. The second you say, I have a 340/360, the voice of opinion is almost always use a 750.

Now the main thing with any engine is what ever carb is on top, the unit must be well tuned a d gone over and over. If the carb is a bit large, a slow mushy feeling is what you'll get. That is the indication that the carb is to large and or the carb is opening up to much for the engine to use it all. Other factors can be involved such as bad/poor timing, too big of a tire, gear ratio being numerically to low, cam to big and the wrong intake manifold.

The beauty of a carb being slightly small is crazy crisp response, stronger signal to the carb is the reason.

Unless the build is a crazy one, your not looking at a big HP difference between the two carbs. I'd bet less the 15 HP on the big end.

Try the dual plane, you might like it even better with the added torque it gives. It may very well help since you have 3.55's.