I see bobl has an 8 stack on his car as well - he might have a better idea on this: what does the 8 stack require for fuel pressure? Do you have an in line fuel pressure regulator as well the the gauge, is the regulator working/set properly, fuel filter in good condition? When you get to 3,500 RPM under load does the engine surge/flutter or is it a constant 3,500 without misfire.

Mechanically; a compression test might reveal valve train anomalies - valve issues might show up initially on a plug. (dry - wet - burnt)