So, I'm no carb expert, but I was thinking along the lines of 496chevy3100. If this was my truck, I'd look to stiffen the secondary diaphragm spring. It's really easy and quick to do, not to mention cheap. A REALLY stiff spring would virtually lock the secondaries out. I think driving on the primaries only might be a pretty good test. If the bog is because too much air is entering at low RPM when the secondaries open, tuning with this simple spring might also offset the rich condition that the OP says he is experiencing.