Welcome to TRS
Symptoms of lower fuel pressure would be losing power at highway speeds or going up hills, situations that have highest fuel demands for the computer/engine
And Lean codes would come up because of this
But dirty fuel filter would also cause similar symptoms
And Fuel Pressure regulator(FPR) on the engine can also cause this lower pressure
Fuel pressure gauge is basically the same as a tire pressure gauge, same fitting, schrader valve
Should run 30-35psi with engine running, 40-45 with Vacuum line on FPR unplugged
look here:
https://www.therangerstation.com/tech_library/OBD_I.shtml
In 1986 your OBD connector should be on passenger side of engine bay near firewall
Looks like second drawing down in above link
Fuel pump slot is labelled in that drawing
If you put a GROUNDED wire in that slot the fuel pump will run full time when key is on, instead of just for 2 seconds
Doesn't hurt anything
You can then unhook the fuel line coming from the gas tank at the high pressure pump(in the frame rail)
Put a container under it and turn on the key, run it for 30 seconds to see how much fuel comes out
Fuel pumps are rated in GPH(gallons per hour), well newer ones are rated in Liters per hour(LPH), lol