Yes a fuel pressure guage would attach at the fuel rail on the driver's side where you can unscrew the cap and access the valve. You can go ahead and get a gauage to mount permanently or just go to your local parts store and get a fuel pressure gauage like a mechanic would have, where you just hook it to the car, set your pressure, and take it back off. That's what I have. To get a reading just screw it onto the fuel rail and start the car and the gauge will shoot up and read a pressure. Pull the vacuum line off of the regulator and then use the adjuster to set fuel pressure to 42-43. Then slip the vacuum hose back on.
No you can't make adjustments through the ScanMaster itself but you can make adjustments with the TurboTweak chip and the ScanMaster will let you see the results (02 sensor reading, detonation counts, etc). ScanMaster just lets you monitor data that the ECM sees.
You ask if 42lbs fuel pressure will make a difference. What kind of difference?