Glenn, is it possible that in the switch over from a V-6 to a 350 the fuel lines got reversed? Ie: you are now using the fuel return line for the fuel flow to you motor and the actual fuel in line as a return line.

If that were the case, it would definitely stop drawing fuel on a hill as the factory return line doesn't go far down in the tank. So it's not going to draw on a hill.

Just a thought. I'm not saying that is definitely the answer. Just food for thought.