More than likely, you'll have to run a wire from the "capped" plug which comes off the negative side of the coil. That will power the tach, as many have mentioned already. As my memory recalls, I had to swap the location of one of the other wires (repin it) for maybe the temperature guage. Does sound right to those of you who have done the swap? I know I had to repin one of the wires... AND YES - please make sure you use the proper oil pressure sender, not the idiot light switch type...
Ken