Maybe you could switch to a fuel pump thats made for a carbureted motor? Maybe in the Summit Racing catalog?
the mechanical should pull the fuel from the tank, i did the same thing in an s-10 and i just unplugged my fuel pump at the tank. one thing you could do though is a fuel pressure regulator before the mechanical fuel pump and it should be fine. or just block off the mechanical fuel pump on the block and just run the factory electric fuel pump with a fuel pressure regulator set to about 6-8 lbs. of fuel pressure.