I think your fuel line should be 5/16 inch steel tubing. You could also have a bottleneck at the tank outlet where scale, varnish, rust, and crud tends to cluster. A good test: Rock the truck from side to side to spread around the imagined junk. If this results in improved performance, time to pull and boil out the fuel tank. Do you have a good inline fuel filter? That, plus a new fuel line (copper is not a safe material), and a tank boilout would help a lot. Find a local radiator shop that does fuel tanks, it is a good investment. grin