[T and S]

I have a 390 cfm holley on a 250 straight 6 engine, carb is new. Engine is fresh rebuild - 9.1 compression, mild comp cam, offy intake, 292 exhaust manifold(2.5 outlet), HEI distributor.
I have trouble with a stumble - letting the clutch out and taking off, it takes off ok, stumbles, then takes off again. I have checked and set float level, mixture screws are at 1.5 turns out, good idle.
It came with .025 accelerator pump nozzles. Have tried .031's and .035's, and have .037's on the way. Still has the stumble, maybe not quite as bad.
Anybody running the 390, have you had this problem, and what nozzles did you end up with ??
Kind of getting frustrated, even thought of trying an edelbrock carb.
Have heard it mentioned this could also be an distributor advance problem - any comment ?? Any help would be greatly appreciated. Thanks in advance !