I need to figure out a way to connect a fuel pressure gauge to monitor the fuel pressure with the engine running.
I came up with an idea and I am posting it here for your review/go ahead. What are your thoughts?

I want to cut the rubber line as shown in the photo and use a barbed "T" fitting similar to the one in the next photo.

The brass adapter will hold the schraeder valve to which the gauge will attach. Once the "T" is installed, it will not be removed.

 

I drilled a hole in the large boss on the fuel rail where the high pressure fuel line attaches. I drilled the hole for a 1/8 inch pipe fitting so I could attach my fuel pressure gauge line to that using a 1/8 inch pipe fitting. Then once I was done with the testing, I just plugged the hole with a 1/8 inch pipe plug.

I can take a photo of this tomorrow and post it here if you need to see more details of how I did this.

 

I did mine this way. Found out that my lean mixture was due to low fuel pressure. 27 psi with or without vacuum at the FPR. I used a pair of pliers and blocked the return line and the fuel pressure remained the same.
I replaced the fuel filter and the pressure dropped to 22 psi and fluctuating. The reading as seen on the gauge is with the engine not running.
I ordered a fuel pump which I will replace tomorrow.
The fuel line where the gauge is attached should have the same fuel pressure as the fuel rail before the FPR.
I will post the results of the fuel pressure after I replace the pump.

 

As part of my efforts to diagnose my non-running 1997 XJ6, I decided I really needed to get an accurate fuel pressure reading from the system. So After examining the situation and finding no good way to connect a pressure gauge into the system, I looked carefully at the fuel rail and decided to drill and tap a hole to a 1/8 NPT thread size. I will then install this modified fuel rail onto the car in place of the original one that is currently in place and then connect up my fuel pressure test gauge into this tapped hole.

See photos for the location and position of the tapped hole and installed pipe plug.

hole in end of rail

plug installed

another view of the plug

 

My plan was also to experiment with another fuel rail but when I went back to the JY to car was gone.
I opted for cutting the rubber supply line and installing two stepped barbed fittings with a "T" and installed the gauge.
After pulling the tank and replacing the pump and in-tank filter all is now well.
After I am finished doing all the work in the engine bay, I can remove the gauge and install a plug like you did.

pressure before pump replacement




fuel pressure at idle after pump replacement

fuel pressure after sitting overnight