It sounds like a fuel problem. Check your fuel line for sharp bends (try to avoid 90 degree elbows) Is the pick up tube in the tank 3/8 as well? Double check your float level too. Seems like your car should be running at least mid 12's
There is only 1 90 degree bend from the regulator to carb.... 3/8" including tank sending unit! If it was a fuel issue, would it not stumble? There is no stumble just flattens out on power!
