OK it's my turn to jump in on this thread, lol.
I had a similar problem with my fire engine, no power at high rpm. Mine turned out to be a loose main vacuum line that looked good until I bumped it and realized it wasn't clamped tight.
You could also check the vacuum advance with your timing light. Connect it up to #1 and rev the engine up, you should see your timing mark move away from the pointer by 20 degrees (about 1-1/2") or more. If no change, hook a vacuum test pump up to the vacuum advance linkage at the distrubuter and see if it moved the base plate the points are mounted on with the engine off.
Mike B
