Where do I hook up a mechanical oil pressure gauge?

Hello, I am Danny.

Just a suggestion. At the rear of the engine, on the top is the oil pressure sensor. You could take sensor out and replace with a Y-type pipe fitting and put sensor back in and plumb in a mechanical gauge as well. Just an idea instead of drilling. Hope this helps and thanks for using 2CarPros.
Danny-

I did that but I question the gauges accuracy in that location. The oil pressure switch is designed to electronically send the message from its reading in that location but the mechanical guage is based on PSI in the line attached.

Hello again.

I have attached two pictures. If you follow the Upper cooler line from radiator (circled) to the adapter block that mounts to the pan that will be the pressure side of the two. Hope this helps.Thanks again for using 2CarPros.
Danny-

Thanks I figured the return line would be the best place to get a reading
it made more sense to measure the oil pressure going into the motor rather than the pressure of the oil leaving the motor.

Hello again.

Yes, you would get a higher P.S.I. reading under pressure than at the return. Hope this helps.Let us know how things work out. Be careful when drilling/tapping with the shavings.Thanks again for using 2Carpros.
Danny-