The OEM oil pressure sensor is on the oil pump assembly near the crankshaft pulley on the back side of the engine. Here is a picture from when I pulled my blown engine out. You can see the sensor plug below the aluminum bracket.
Hmm from what it looks like there wouldent be much room for a "T" there for a gauge sensor? Do you know if there was mu h room. Etwewn the firewall and that sensor???