Zero to thirty pound liquid filled gauge would meet your needs. Harbour freight etc probably 10.00 or less. You are correct in that it would give you early warning.

It is not good on these engines in my opinion to run with less than 10 pounds pressure present in the injection pump. They will run though. You cannot really monitor just the lift pump pressure because the relief valve on the injection pump is preset at whatever it is now with time and age.

Idealy it should be set at 19 pounds. In your case your earliest warning might be the pressure sagging off at speed as the filters start to load up and cannot pass the increased volume of fuel. Instead acting as a major obstruction to fuel flow and dropping the pressure in the base of the injection pump.

If you go for it it will meet your needs in my opinion. I still think the addition of a gauge to the base of the injection pump is a good ideal for any of these old diesels for the same reasons as yours. Low injection pump base pump pressures causes issues like poorer milage. I would also plumb the gauge back into a place like the glove compartment..Or perhaps out in the open if a neat arrangement can be done.

That addition not only gives you advance notice a fuel filter change is coming up. It allows you some room to do it at your convienience before the engine quits. Also running any fuel it will aid in troubleshooting down the road perhaps. Even indicating other deteriorating conditions. I have always though it is worth the effort and minimal cost in todays world to have one installed.