one thing that helped me a lot when using the 484 years ago was to use lighter springs in the distributor and disconnecting the vaccuum advance. they are mopar performance p2932675 and cost less than 20 bucks. use one of the springs in the kit and retain the smallest of the 2 already in the distributor. also, tailoring the shooter sizes in the carburator helped a LOT too. btw the 750 holley is very well suited for the 340, be it double pumper or not. you just need a little bit of fine-tuning, thats all .good luck.