Terry,
Here is a great explanation of the percolation problem from Walt Hane's web site:
http://eps-hane.com/techtips5.html
I had a 351/393 with the air gap manifold and experienced the same problem. I took Walt's advice and used a 30% 110 race fuel / 91 pump unleaded mix when winter blend gas was at the pumps. I also had a Holley blue electric pump and once observed the winter fuel boiling in the fuel filter between the pump and the tank at the rear of the car! Apparently the pump gears were creating enough heat to boil the winter fuel. Oddly enough, I have not experienced the percolation problem with my 428 - go figure! Not everyone up here has had the problem. It doesn't happen to me at the track with summer temps in the 90s.