I'm not sure about your harness question.
This is a swap that is do-able though. You just have to replace the oil pressure sending unit with one for the Rally gauges. The standard dash had a light, while the Rally gauges had a meter. You also need a power wire and signal wire for the tach. As I recall, the power wire was blue, with a cap on the end to fit over a screw on the back of the tach. The signal wire comes from the "-" side of the coil and is gray. It has a lug that slips over a terminal on the back of the tach too.
Mike