I don't remember if you told us what type of intake (model and single or dual plane) was on your 340. I've never been a fan of the Holley 600 vacuum secondary carbs, but given that you have a brand new in the box one available, I'd swap it out to eliminate the carb as a culprit to the poor performance. I'm assuming both carbs are vacuum secondary and not double pumpers? A lot of people will disagree, but I always lean towards a double pumper Holley for a car tuned for performance, especially on a single plane intake. Properly set up they will run circles around vacuum secondary carbs. I think you need to optimize the combo you have and then determine what you want to with the car. As mentioned before, make sure your getting the right amount of timing and fuel delivery. The stock vacuum secondary spring (black, I believe) may not even allow the secondaries to open fully, which will definitely kill acceleration.