goto a local tuner with a dyno and some wideband o2's and figure it out, no one can tell what fuel pressure to set your car to. Mustangs came stock with 36-40lbs from the factory. If i had to guess i'd say 38lbs
Set it at 38/40 psi with the vacuum off and plugged - engine warm and idling. That's what the computer expects. Modest changes in pressure are compensated for over time by the computer anyway. If you turn it up for example to richen the mixture, the O2's will read the richer mixture, and they'll shorten the pulse width to reduce the amount of fuel and get the mixture back to the range where the computer wants it. And the computer will remember that it had to do that, and over time will store those new values and use them going forward. Same thing happens if you lower the pressure - if it's leaner than the computer wants it will increase the pulse width to richen it back up. So set it where the computer expects it to be - the stock setting. If you really want to optimize mixture/power, as bmo said you need a wide band a/f ratio dyno run and a chip/tuner/tweecer to alter things inside the computer. The computer "learns" about fuel pressure changes pretty quickly and compensates for them.
Get one of your buddies to ride along with you. Find a level stretch of road where you can do some testing. Don't choose a speed range that will make you shift gears, the shift quality & speed will affect your results. Use 2nd or 3rd gear and do a run from 2000 RPM to 5800 RPM. Call out the RPM as it increases and have you buddy record it and the elapsed time. If you have one of those cheap digital watches like I do, a stop watch is built in. Start the fuel pressure at 36 PSI, vacuum off and make a run. Turn it up to 37 and make another. Be sure to reconnect the vacuum every time you set the pressure. Keep turning up the pressure and making runs until you notice a drop off in time. If your bud was careful to record the results, you'll have a nice chart to use to find the optimum pressure. Note that this only affects the WOT performance, for part throttle, Michael's comments apply.