Mark
The 70s 4/4 had a relatively simple fuel system so diagnosis should be straightforward.
I would first check that all the unions on the fuel lines are tight and not pulling air. Then perhaps check the obvious items like the float chambers, the carb jets, and even the fuel in the tank.
Then I would rig up a temporary fuel feed from a separate container directly into the fuel chamber, if this enables the engine to fire up and continue to run , you know that the fault lies backward of the this.
Are you using fuel with ethanol in it? If so this can soften rubber and gasket material used in the 70s , and this can lead to leaks and blockages. Try looking at all the rubber components.
Now look at the pump itself , is it a diaphragm type (sorry I don't know) and if so check that there is no tear in it, if so it needs replacing.
If you access it easily , check that the fuel pick up tube in the tank has not picked up something which is causing a partial blockage. You can do this by blowing compressed air back through the line , but make sure that you vent the tank.
Whilst on vents , check that the petrol cap vent is not blocked.
Good luck
