850 Carter Thermoquad flooding at low rpm

Check the temp of the fuel pump , intake and carb base with a laser thermometer after the car starts to stuble. I've had to make thicker insulating type fuel pump and carb gaskets for a few builds because of ethanol boiling off in the pump at around 173 degrees F.
My bet is Heat soak in the fuel system somewhere. Probably at the fuel pump itself heated by the header tube. It would explain why it quits stumbeling when you get going because it's cooling off via air flow.
Headerwrap around the tube or a heat shield around the pump.
I ended up finding this on mine by simply pouring room temp bottled water on my fuel pump when it was doing the same one day. Once I got it home and measured my fuel pump temp it was 240F even though the block and head were 180.