I'm proud and happy I figured out this issue with the carburetor.
The engine runs beautiful now !
My problem was a too stiff spring inside the Rochester B power valve piston.
You could punch the gas pedal without any hesitation, but if you left it too wide open, the engine eventually started sputtering until you backed off the gas.
After eliminating all other factors, I realized the power valve piston must be too open at low manifold vacuum, supplying too much gas.
So I went through my old Rochester B parts collection and found a lighter spring.
Amazing what the right little spring can do.
I can push the gas as hard as I want now and the engine responds very well.
My theory was correct and the solution worked - feels great!
Anyways - just posting this in case somebody has symptoms like I had.
