Mine was doing that, except it would start at 40psi, then then work it'* way up, spiking occasionally until it pegged for good. Turned out to be the oil pressure sending unit. I just unplugged the wire to it and then plugged it back in, and that solved most of my problems, but I have a new one to go in (thanks bill) when I get a chance. If you need to change it, I found it is easiest to get to if you remove the passenger wheel and the splash shield behing the wheel (not the plastic inner fender). Not sure how hard that is to remove since I didn't have one on my car after the engine swap. But with it removed, you can access it easy.
You can kinda see where I am talking about in this picture:
I believe there is supposed to be a splash shield above where the jack stand is and the Oil Pressure Sensor is right behind the strut on the motor.