Holley 4150 setup (PLEASE HELP)

[mhouston]

I can't for the life of me get my 345 to run correctly. Idle is fine, but theres a bad stumble when you first start to open the trottle, and then a bad flat spot at about 1/3 throttle. I know its an issue with the carb, so I need help for how to set this thing up right once and for all. It's a aftermarket Holley 4150, 650 cfm, DUAL FEED, electric choke, vacuum secondaries. I hear this is a difficult carb to set up on a SII because of the dual fuel inlets, but its what I've got and I can't afford to change it. Some other stuff:

Compression 140 lb on all 8
vacuum 18 inches Hg at idle
the whole ignition system is new, including a NOS prestolite POINTS distributor
timing set at 8* btdc, vacuum advance works perfectly

I'm tired of fooling around with this Holley and not being able to get it tuned right, hope ya'll can help. Been driving it like this for about a year...it'd be nice if it ran right.

EDIT: The carb is a 570 cfm Holley street avenger P/N 80570.