1974 chevy truck with 454 in it...

Can you take air horn off carb w/o removing carb, any gas in the float bowl? Maybe you need a carb kit? If you take fuel supply line loose from carb, hopefully you can aim the loose end into container, bump the starter, any gas come out? If not, have to double check mechanical fuel pump installation. If I remember correctly, there is a rod in fuel pump cavity that operates the fuel pump arm, correct? The rod and arm are lined up, correctly? The fuel line/s from gas tank, at the pump, take them loose, blow through them, can you hear air bubbling in gas tank? If so, then I think the lines are clear. good luck

The fuel line may be rotted out where it goes over the rear axle , it won't leak at the top it'll just suck air , in the old MOPARS the rod or internal cam for the fuel pump would wear out