if youre dash was the standard "5 holer" there isnt a wire for the tach in that harness that i ever found. simply run the wires from the back of the tach (they are, or should be marked) to the appropriate places.
as far as the oil pressure gauge, you have switched from an idiot light to a gauge. the sending unit will NOT work. the idiot light sender is simply an on/off switch, the gauge sender is more like a reostat (like a slot car controller) change the sender and locate the wire in the dash pod plug, follow the lead in the printed circuit down to the corresponding pin in the plug and connect it.
sounds like a bitch, but it isnt toooo bad.
paul