Do all Fuel Pressure guages HAVE to be mounted on the outside of the car? Do people actuall mount them on the inside? Seems dangerous.
Also where is the best place to fit the T coupler I was thinking just after the Fuel filter.

 

To be honest with you, more than half of the peeps here have an interior mounted fuel pressure gauge that can be used to monitor the FP while driving. They are electrical gauges, meaning that they have a sender unit on the fuel lines that sends the FP info via wires to the gauge inside the car, so no fuel comes pass the firewall. You are probably thinking about mechanical gauges.

As for mounting a T fitting, just go to Lowes/Home Depot/Ace Hardware and get some brass fittings and make your own. That is what everyone here does. And you are right, install it after the fuel filter.

I hope this helps you, post back if you have more questions.