Mechanical speedo will be the long shaft, electronic speedo is the short shaft. How to tell? If it's an electrical connector with 3 wires hooked up it ain't mechanical. If it's got the sleeved cable coming off it ain't electronic. I think the short shaft speedo came about in the early 90's with the proliferation of computerization, but at any rate I'd be willing to wager a small confectionary yours is the mechanical speedo that will use the long shaft.
Mike
Oh, here's a pic showing the diff.....