Don't know how yours is, but on mine the speedo cable goes from the instrument panel to the speedometer adapter (which has a ratio stamped on it). Changing the speedometer adapter is what calibrates a mechanical speedo. It screws on to the transfer case, and is coupled to the speedo gear with a little pin that has a square shaft on one end and a round one with a little tab on the other end (the most common speedo failure seems to be when that square shaft becomes rounded).
I don't know what your setup looks like, but even an electric one starts oune with some mechanical motion somewhere on some level. Might be able to retrofit something, or just tear out your entire system and put an old one in.
As far as your conversion goes, I'm just finishing that one up, and have a gallery below in my signature. Will have more pics this weekend. Let me know if you have any questions.