anyone know how to rig up rpm gage???

[518]

Hi, i own a 79 518 and was wondering how to hook up a rpm gage. I currently have the clock next to the speed instead of the rpm gage and would like to change them. maybe one mounted on the dash board and leave the clock alone or replace it with a cluster from a bimmer that already has one. If i obtained a gage, how would i proceed to hook it up???? any advice greatly appreciated. thanks