This thread pretty much gave you the road map for figuring out what is wrong.
http://67-72chevytrucks.com/vboard/s...d.php?t=633007
Go back and review and test the system from your plastic tank to the carb, opening each fitting in sequence: line from tank to pump, pump outlet, fuel line to carb. Also make sure that the carb bowl is primed, if is has been sitting for a while without running the gas has evaporated out the vent, saves time and battery and starter life not having to grind away while the pump fills the float bowl.