Anyone know where to hook the line up for a mechanical oil pressure gauge on a 90 turbo car with the external oil cooler?

There should be a removable plug that should allow for it at the filter housing. Worse case would be to cut one of the oil cooler lines and install a fitting for it. They make those fittings in -6 & -8 sizes.

Good luck getting the plug out. It is a allen head on ~1/2 pipe plug. I stripped mine while using a torch to heat it up. When my car is back together it will make a fun project for qpr ;)