Why do you want to T it off? so you can still relieve the pressure in the system at the other side of the T? I have an electric fuel pressure gauge and from my experience you could do it right now and not necessarily when you do the LIM.
1. They always recommend to unhook the neg. battery cable when working with your fuel system just to avoid that one freak accident where you have sparks and fuel meet.
2. relieve pressure in the fuel system. I use a towel over the pressure relief valve and then depress the pin. The towel catches the fuel that drips/squirts out.
3. you need a schraeder valve tool sometimes available in the bicycling section of stores or the car section will have them as well. The tool is used to take the core out of the schraeder valve on your fuel rail.
4. Then you just need the right adapters to connect your fuel rail to the T to the gauge. Just make sure to use a sealing compound on the threads to avoid leaks.
I remote mounted my pressure sensor using SS line from Autometer. This isn't like what you want to do but if you change your mind and want the fuel pressure gauge in the car this is how i set mine up.