The port most people use is on top of the engine just behind the distributor. There should already be a sender switch screwed in there with a wire coming off it. That’s your oil pressure (green) light. To get a proper gauge you T that port. Screw the idiot light sender into one hole and the gauge sender (either oil temp or oil pressure) into the other. Then wire it up as instructed by the gauge maker.
Note: T-ing that port will cause the oil pressure light to go on slightly earlier than it would ordinarily. It’s set to light at like 5 pounds or less, with the port shared it might blink on at 6-8 psi. Which is fairly normal at hot idle in old, stock engines.
Early MG TDs had an oil pressure gauge, later ones came equipped with a dual gauge reading oil pressure and coolant temperature. These are old school, requiring a capillary tube from the engine’s oil port and a wet bulb to be secured somewhere in the coolant system.