there is also a schrader valve on the back of the fuel filter housing. Yes the cap would work too. or you can use a tapped banjo fitting on the outlet of the filter. There are many choices.
I used the banjo on the outlet, fuel pressure is 16 at idle and can drag lower than 12 with the stock lift pump in the stock location
running catcher ecm and mach 2. 5 injectors.
hope this helps