Hi James,
OK, it have been a couple of decades since I fitted one, but here goes.
Ideally you need a capilliary fed one that taps into the oil system and
then has a small pipe that feeds the oil to the pressure guage.
I suspect that your engine will have an electronic sensor somewhere
monitoring the oil pressure - the easiest solution is to plumb in a T
piece between the sensor and the block and run the pressure guage from
there. You will need to locate the sensor and remove it in order to
track down the appropriate fittings.
If I have time later today I'll dig around my haynes manual that also
covers your model and see what else I can dig out.
--
Regards,
Julian Voelcker
Mobile: 07971 540362
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift