Ive got a freshly rebuilt (less than 2 years old) 77 302 in my 63. Mild build, cam edelbrock intake, holley 600 cfm carb, headers (home built so not the best). I just got the carb tuned up and the motor just got a new MSD dizzy, coil and wires. So far this combination has been awesome but theres one problem i have with it. If i get on it from a standing stop or while cruising on the freeway (2500 rpm or so) it hesitates for a second before accelerating. This hesitation gets worse the slower im driving.

Does anyone know what could fix the hesitation issue? My guesses are the carb is too big or maybe the carb spacer is causing issues (not a huge fan of the spacer it whistles at certain throttle positions). Anyone have any ideas or advice??

 

turn up your accelarator pump on your carb

 

First of all the 600 is fine on a 302 for street driving, although I wouldn't go any bigger, unless you're drag racing at 7000 rpm.
Does the new distributor have vacuum advance or internal advance weights? If so, make sure the weights are free and don't stick and test the vacuum advance with a handheld vacuum pump to make sure it operates properly and holds vacuum. What is your initial timing set at?
Make sure the accelerator pump linkage is adjusted properly. The accelerator pump arm should begin to move the second the the throttle linkage begins to move.
The whistling your hear could also be trash clogging the idle air bleeds. Try some areosol carb cleaner with the little straw to flush them out.