Holley Street Avenger on 408 / 416

As 72BLUNBLU said in post #2 explain the stumble more in detail (does it hesitate when you just give it a little gas or does it hesitate when you floor it, etc) and a complete run down of your engine build sure would help. If your going to ask a technical question please have the courtesy to reply to questions asked to you

Nevertheless here's a quick run down of things that can cause a stumble.

Lean jetting

Too small of a squirter or a lazy pump cam for the accelerator pump

Too loose a spring on the vacuum secondary's

Improperly adjusted floats (generally set too low which I've seen from the factory)

Every engine is different and can require specific tuning

A buddy of mine bought a 770 avenger brand new and out of the box it was a mess. Floats set way too low. Electric choke set so tight it wouldn't come off all the way until the engine was fully warmed up and by then it had the plugs fowled. Looked like a 5th grader set it up. After I went through it and adjusted everything properly and installed a stiffer spring on the secondary's it ran great