I know you need a sending unit in order to make the conversion but does the sending unit replace the speedometer cable or are they used in conjunction with each other?
Generally, you replace the speedo cable with the sending unit. You can use the vehicle speed sensor (VSS) from a '94-5 T-5 (that's what I used - may be applicable to other years as well) to drive your electronic speedo. The sensor has a pass-through for a speedo cable, so you could actually use both if you wanted. I capped off the speedo cable opening and just use the speed signal (simple 2-wire connection).