First off what yr is your car.. The easies way to install the oil pressure guage is if you have a Romeo Block.. One way to tell is jack up the Stang and look at the unit to there the oil filter connects to. If you have what looks to be a plug at the very bottom then that is where to plug in for the oil pressure guage that also means you have a Romeo block..
Also on another note I would highly recommand to get the copper tubing for the oil pressure guage and to buy some red RTV to sell the threads. Just my 2 cents I just did this last week and learned alot of things.