my oil pressure guage on my 85 12a is reading high - need to check its accuracy - how do I connect an external oil pressure guage?

 

Take the factory one out and put yours in that location. Its the large one next to your oil fill on the block. It will be goldish in color.

 

its actually on the rear side plate under the oil FILTER ( im sure thats what john64 meant).
it will have a plastic slide on terminal that is attached on the end of it. it slides on from the side....remove the wire and grab a 14mm (i think it is) and put your new mechanical in there to verify pressure