Just a note on cheap mechanical gages. Having an instrumentation background, I don't like cheap ones much. They are usually out of calibration sometimes as much as 7-8 pound. However, you are looking for a constant pressure and even if they are out of cal the indication may tell you what you need to know. Just be aware that there may be a (+ or -) 7psi deviation. If your readings are close to specs, then you might get a known good gage before you jump into anything major. I do like checking with a mechanical gage though, as much as I love Mustangs I'll agree with one response above that the factory gages are not very good.
If you find that the pressure is actually low, and you are on a budget, then I would suggest pulling the pan and removing the caps off the mains and rods to inspect the condition of the journals. I have on two occasions found high mileage Mustangs with very little wear on the crank. My convertible had a very smooth crank and the mains and rod bearing were worn to the copper, which was the main reason for a low oil pressure reading that I was receiving. I replaced the bearings in the car with factory standard bearings. I plasti-gaged each one to check clearance and every one was in specs. The rear main was at the very limit but within, the rest of the bearings were well within.
I also replaced the oil pump with a (HV) high volume unit since I figured the cam bearings were worn too and I wasn't going to replace them. As it turned out the HV pump has an overpressure relief at 75psi. I checked the pressure with a good gage and when the engine started the pressure went up to the 75 limit and the excess was dumped as indicated by a constant 75 psi reading. I was pleasantly surprised with 127K on the engine. That was six year ago.
