I have a very similar unit. On mine, the gauge goes right into the side. I can't really tell from the pictures, does one of those plates with the barb fittings have the slots exactly in line with the barb fitting? If so, that's the one you'll use. You'll pull off the stock FPR, and put that fitting on in its place. Put some black RTV lightly around the edges to get a good seal. Then cut the fuel return line, or get some extra return line, depending on where you want to mount the FPR.
The small barb fitting is for the vac, the large fitting directly opposite the adjustment screw, or pointing toward the top of your pic, goes down and takes fuel away from the regulator, and the fitting that is pointing left in your pic has a line going from it to the fuel rail.
Here's a pic:
Edit: Is there a place for the gauge to screw into the regulator? If not, you'll want to put the gauge in the fuel feed line, so you can effectively measure the pressure that's going into the rail.
