whoa buddy i dunno where you heard that but everyone ive talked to said that a tach cluster will plug right into a non tach truck. theres like 18 wires (?? someone correct me??) in the harness, one of them is the tach, i want to say brown but i cant remember. ive never heard that it wasnt simply plug and play, do a search, you'll see what im talking about.
edit: as an afterthought the only thing aside from mileage you'll have a problem with is if you get the tach from a truck with a different engine. the 4.3L trucks redline differently then the 2.2's so the tach reading will be off unless you get the cluster from the same type of truck.