Had an issue years ago with a 340 in a 70 Challenger. The engine would start and run just fine up to around 3500 rpm where it would shudder and cut off. Took a while to figure out, but the fuel line on the tank end was shot and had been temporarily replaced with about two feet of rubber line. The force of the suction from the pump was actually causing the rubber line to collapse and cut off the fuel. Not saying that's your problem but many times it's something stupid.