39 psi with vacuum off. With vacuum on it should rise up a few psi under load.
Only reason i would have one on a car without a reg is to see if you were running out of fuel at the end of a run. I had a detonation problem at the end of my 1/4 mile run. After watching the fuel pressure drop off 5psi towards the end of 3rd gear i discovered my fuel pump was crapping out on me.
Other than that there is no reason without a AFPR