Hi Max,
The most likely culprit noticeable from the info you gave is the Warrior intake - my understanding is that this is a rather large runner single-plane intake, intended for use with heavily ported heads in a higher RPM range (especially on a 400). The stock heads become very flow restrictive on a 400 (in my experience) around 5500 RPM.
Also, the dominator carburetor is a double-pumper, mechanical secondaries, race oriented carb. Your GTO is a heavy car, and I am not sure what gearing you have - but you may very well be better off with a vacuum secondaries carb.
I would suggest trying an Edelbrock performer or factory ram air type intake manifold with the vac secondaries carb (or try a good Q-jet). A good comprimise for the rear end gearing for a decent launch with decent top end would be 3.55's. If the converter is a good 2500 RPM stall converter, it should be fine.
I am not sure of the cam specs either, but a RAIII spec cam should run well in your engine.
Definitely check the ignition advance curve as well (use a timing light and 'map' it out versus engine speed to see what the mechanical advance is).
Best of luck!
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