All motor trouble shooting starts with the basics;
1). FUEL
2). AIR
3). SPARK

2 & 3 seem to be covered as the motor runs when primed with fuel. Going back to the original post, he states that there was deterioration at the rubber seal to the cap, the truck ran for several hundred miles then started giving trouble requiring the foot to be held firmly into the pedal and this got him home.

At this point logic brings us to the carburetor (72 pre-fuel injection), one would assume the carb is plugged up with debris from methanol fuel blend we our now using (a likely suspect to the original rubber components deterioration).

Pressure at the carb only tells us the fuel is arriving, not how much is getting into the motor as the motor is not running long enough to see a pressure drop (you can have pressure without volume).

With the knowledge above we would assume the rubber components are likely causing debris to enter the filters and carb, but the deterioration causing the debris may not be limited to just the rubber hoses, as the fuel pump and carb itself have internal rubber components.

I agree with the advice given that all components need to be examined for a long term fix, otherwise you will be further frustrated down the road with continuing problems.