on the note of your manifold-- what are you running, like a holley strip dominator?
High rise single plane intakes are generally for higher RPM's only. They are horrible for city driving and on a motor, they tend to starve cylinders 7 and 8 on fuel. The only way they come into play is if you can really get the rest of the motor flowing. (heads and a high duration cam) which eventually leads to none streetability. On your setup, I would reccomend going with something like a weiands stealth (
http://store.summitracing.com/ part # WND-8016)
I would also reccomend putting some more voltage to your plugs-- try getting an HEI coil. But mostly, I believe your problem is your intake, the motor just can't flow enough for what it wants to dump in. Good luck!