http://duster318.freeservers.com/tech/VSS/universalvss.htm
Here's a pretty informative article on how to install a driveshaft mounted speedometer sensor for not much money. I've been looking at some more modern transmissions and some don't have speedometer senders in them anymore, or if there isn't enough room in your tunnel for it to fit, this may be the answer.
I wish I'd have seen this before I cut an access hole in my tunnel and made a cover for it!