We've had good luck with the 600's on the smaller motors, 750's on the bigger stuff. In years past I watched guys with sticks take the air door completely out. Stock jetting is very close. You may want to swap the step-up springs so the metering rods stay seated in the jet at idle. I think that these carbs biggest down fall is there transition from idle/leave rpm to WOT. Whether it is lack of pump, air door to quick/slow I haven't figured out yet. They're not terrible but there is room for improvement. I see a few AVS but they don't appear to be any better. Even the 800's on a big motor. My 572 A/FX car runs 2 750's at 113 square jetting. I've moved it around a couple of times with minimal change. My buddy's 440 (N/SS D) runs stock 600's and stock jet, same deal runs about the same. With a stick I'd use the two step in a heart beat. I can't with an automatic or I would.
Doug