I have the HSR with Holley fuel rail & adjustable regulator. Can I just unscrew the Schrader valve & install the gauge in that hole (near the passenger front) or do I need to plumb it so I still have a Schrader valve?

 

Im not 100% sure but im pretty sure you just screw it onto the shrader valve, just depends on the type of gauge connection im guessing, either way if it fits then its fine as the shrader valve is just to release the gas from rails its not important lol.

 

Either way.
Can remove the fitting and install gauge direct to the rail
or remove the valve from the fitting and use a -4AN gauge adapter to mount the gauge on the Schrader fitting.

The Schrader fitting is a leak proof pressure test point ,the test gauge pushes opens the valve inside to check the pressure without spilling fuel everywhere.

A reg pressure gauge doesn't do that ,so if you don't remove the valve the gauge won't work

 

Ya what vetteoz said lol didnt think about that but ya a schrader valve is the same thing on a car or bike tire so you use the same tool and remove it then connect gauge with rite adapter or un screw the nut that sits on rails and screw in there. But if your gauge is a tester then you dont have to remove valve.