I'd disagree with installing a electric fuel pump. I'd make sure that you have the correct mechanical fuel pump on your engine. Does your truck have a charcoal canister to vent the fuel system? I'm not sure what year trucks went to a 3 line fuel pump. Its possible that you are not allowing the fuel system to vent? Sealed fuel cap and then a return line going to tank and pressurizing the system causing the needle and seat to not hold and letting the fuel run into the crankcase until it bleeds off enough pressure to let the needle and seat to hold the fuel back. Just a idea.
Good Luck