I thought when I did my fuel pressure gauge that I just unscrewed the cap then got a wrench and unscrew the valve out of the fuel rail. Thats what I remember anyway.
Take a rag and put it over the valve with the black cap off. Take a small screwdriver and press on the little stem inside the valve. Make sure the rag is over it!!!! This will relieve the pressure on the fuel rail. Then take a wrench and unscrew it. You'll need an adapter to put your fuel pressure sending unit on it. The threads on the sender don't match the fuel rail. I can't remember what size they are, but any mustang company knows and usually has the part, it's just a couple bucks. Make sure to use gasoline grade teflon tape, so you don't have any leaks. It's the one in the yellow container, not the blue.