What is a GD300?

"I get good fuel flow when priming, to the fuel filter, but very little from the outlet from the IP(on the block side of the IP). Even cracking the fitting, to bypass the check valve doesn't help."

This seems to imply that your the spin-on Fuel Filter is plugged. Or some other obstruction from the filter to the Fuel Injection Pump.

When you pump the hand primer or the Lift Pump is working and the fuel flows into the fuel injection pump housing and back out that is a separate system; the fuel supply system. The Rack has nothing to do with this part of the system.

You should be getting out as much fuel as say the hand primer moves.

You might try taking the fuel supply line at the fuel supply/lift pump and running a longer hose into a container of clean diesel fuel and running long hose from the return back into the container so you can get a really good idea of what is going in an coming out.
If things improve when you do that it may be that there is some restriction in the Fuel Line on back to the fuel tank.

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84 300D, 82 Volvo 244Gl Diesel