Truck starts but shuts off thirty seconds later

I have a 1991 Chevrolet Silverado c1500, 130,000 miles. The engine has been changed from the original 4.3L to a 010 block 350, and a carburetor and HEI ignition has been installed..The in-tank fuel pump is still in, (with a regulator). After the carburetor was put on, the truck ran very well for about a month, but recently it has come up with a problem. The fuel pump cycles on when the key is turned on, the engine will start and run fine, but after about 30 seconds the truck will shut off. Looking at the wiring schematic, the Oil Pressure Sensor works fine, and the Fuel Pressure Relay is new. Is it the pump? or am i right and if it works it works? I'm at the end of my rope and this has stumped quite a few people so far. Help!