Also have had many questions about the speedometer thing....
So, The Dana 300 uses a GM style speedometer cable. The fitting on the transfer case is a 7/8" inch 18 TPI fitting. (Toyota transfer case also use this).
So, you can get a sender like what I have in the picture above. www.summitracing.com
sells them, among other places.
Here's what I've got (mine was included in my big kit, but you can buy them seperately).
Other companies also make them, search in gauge sending units for speedometer senders.