Snowman, remind us of your fuel system. Fuel pump being used. Hose diameters, fittings what size. Some fittings look big on the outside but are internally small.
I know for when I was blowing though my DCOE Weber carbs, I needed the largest needle & seat they offered, other wise it would run out of fuel towards the top of 2nd gear (TH350)
You could install a larger needle & seat for the carb.
As far as in getting lean in the middle & to the end 4th? Sounds like it is running out of fuel. What is the max output pressure of your fuel pump?
Check that you are getting full voltage @ your electric pump, could be dropping down when a load is applied to the pump?
Before this engine really blows up for good, try & do some "G" tech testing, smart phone app or something, this is interesting we would like to hear about, well I know I do anyways.
