Hey Carl - I'm with you. That's why I pulled the jets out the first time. It seems like a lean secondary issue. I removed the jets and air correctors from the emulsion tubes and inspected all parts with a magnifying glass. There was no visible blockage .Next, I tried bigger jets with no improvement. Yes. I have a timing like and I have confirmed that the mechanical and vacuum advance function on the new distributor and the timing is set now with 15 degrees initial advance at idle.My worthless experience is that major problems such as cam timing, ignition timing and even bad compression would show up more as an engine that's hard to start or with a terrible idle. If everything seems fine except for a dead high rpm issue then I wouldn't dig too deep for the source of the problem. I'm hanging my hat on an ignition that does not advance properly or a blocked fuel issue on the secondary.
Chris, I suggested swapping changing jet stacks because it's real easy to do and will instantly indicate if there is a blockage...which could be in the emulsion tube or even air corrector. Just helps in the process of elimination. If you had an O2 sensor and gauge you could watch that go super lean or rich when you hit the "dead zone" but I'm sure you don't have one of those hooked up. Do you have a timing light to check the advance of the ignition?
I will try swapping the primary and secondary main "stack" as you suggest, just to eliminate that. I'll let you know how it goes. I have also been toying with getting a wide band O2 sensor and gauge, but will wait until I've checked the other suggestions in this thread.