The word "Cummins" does not have a "g" in it.
Run a hose from the inlet of the transfer pump into a bucket of clean fuel. If you get fuel pressure that way, you have just shown that there is a restriction in the pickup assembly. Or no fuel in the tank.
Remove the banjo bolt from the side of the VP44 nearest to the front of the truck. It will have a spring and a check ball inside. You can clean it with compressed air and solvent. This is your overflow valve. If the spring is broken or collapsed, fuel will flow freely back to tank.
If the first 2 ideas don't work, put everything back and run your return line into the bucket of clean fuel. If you see lots of air bubbles, you are sucking air on the supply side.
Does the truck run?