[Dave in MN]

If you have 2# of fuel pressure, your problem most likely is with the carbs.
Check the flow of fuel right at the inlet to the carb...you may have an obstruction in the line or at the carb fitting.
Your trouble is not with the engine as it ran well with the updraft carb and stock manifold.
I have run many engines on a test stand with varying carbs, intakes and ignition systems, I have had similar problems. Most of the time the issue is something simple but critical. Hope you find the issue soon. Please report back.
Good Day!