The best place to tap into the engine for an oil pressure gauge is from your remote filter adapter housing. You can remove the housing by disconnecting the two oil lines, then loosening the center fitting which tightens the adapter to the engine. Spin the adapter off and the tap a fitting into there. Re-install the housing and hook up all your lines. Go with a mechanical gauge as well, the electrical gauges go bad all the time due to vibrations screwing up the sending unit. Another place I have seen the gauge hooked up to is using a t-fitting at where the top end oil lines go from the head to the case of the engine.
Todd