There are two possible places on a typical Chevy small block - there is a 1/8" plug on top - behind the intake manifold. There is also a port (1/4" I believe) above the oil filter.
By the way - why do you want to install a mechanical gauge? There is a lot that can go wrong with the hose/tube needed for the gauge. An electronic gauge is just as accurate and is a lot easier to install. Plus no tube to get damaged and lose your oil and make a mess. Unless this is something to simply check your pressure temporarily... I would strongly suggest an electronic gauge.
You can always remove the current oil pressure sender and use that hole. Here's a pic I found online of where the hole is for the top, back of the engine. It will be behind the intake manifold at the back. The brass fitting to the left of the pink thing in the picture below is the location:
only two locations for reading oil pressure on an SBC. the one in the pic above (directly above the oil pump), the other is IF the port is tapped above the oil filter boss on the left side of the motor. BBC in same locations
