Get a 14" or 15" long steel braided line that has male fittings on each side with the same threads as the OEM oil pressure sending unit. You can usually have it made at a fittings shop. Also get a T fitting that has all female fittings. Screw the line into the place where the OEM sending unit connects to the block. Screw the T fitting onto the other end of your steel braided line. Connect your OEM pressure sending unit to one opening on the T and the aftermarket gauge sending unit to the other opening on the T. Make a loop in the line to allow for the vibration of the engine to be diffused by the loop and mount the T and sending units to the firewall. Run your wires and you're all done.