These pumps fail due to bad fuel from sitting, I replace a pump a month from auction trucks.
You have to pull the pump and see if the shaft is even in one piece anymore.
its common for the shaft to shear internally, and you will get the exact symptoms you have.
stanadyne used a very thin diameter shaft on this pump and shearing is very common.
once the pump is removed, rotate the shaft by hand, you should hear the cams and rollers inside, when the shaft shears,
It just spins freely.
Had 2 x 6.2 engines with sticky injection pump here in Australia, wouldn't start with no fuel delivery from the IP.
Still had the lovely army fuel from the US in them, smelt like turps.
Talked to our local fuel injection guy and he commented that most likely the plungers were stuck in the pump and would not return to load fuel.
Asked about the transfer pressure and was told it should be about 120 psi max.
We made a manifold and filled it with fresh diesel and hooked it to the inlet to the fuel filter.
Put air pressure on the manifold about 30 psi and bled clean fuel into the filter.
Raised the pressure to 120 psi and cranked the motor.
It started after about 10 seconds cranking and then stopped.
Did this a few more times and it would idle a bit rough.
Hooked up the fuel lines after draining the tank and refilling with fresh diesel.
Started up ok and ran truck around for 30 minutes and running great.
Bit of a bush fix but may help others as this seems a common problem.
Attached a couple photos of the manifold used, parts came from the local hose and fitting shop.