The fuel rail fitting on the 3.0L is a special fitting that requires an o-ring for a seal. The adapter goes between the hose fitting and the receptacle fitting on the rail. If you have access to a fuel pressure gauge, there might be an easier way to test rail pressure than finding the rail adapter..... get a fuel supply hose from the junkyard (or a new one from the dealer), cut it in the middle and install the required fittings to connect the gauge. Then you can test the pump by clamping shut the return line to the tank from the fuel pressure regulator and see if the fuel pressure rises to 70psi or so (with the engine running at idle). If it does, the fuel pump is probably ok. When the clamped return line is released, rail pressure should drop to ~38psi. If it drops lower, the regulator is faulty. The modified hose could be left in place for future use, but you'd want to make certain it was well sealed/clamped so there are no fuel leaks.

MMC didn't do home mechanics (or even techs) any favor when they didn't design in a fuel rail test port. It would be fairly simple to add one when the fuel rail is off the truck.... cut the crossover pipe and install a tee fitting with a test port... extend it far enough so it's accessible when the engine is back together and it would then be simple to diagnose fuel pressure issues. I think it would be a very worthwhile change to our trucks.

Frank