Which type of VW gearbox are you using? I have a T3 (Vanagon Doka) and the speedo drive is off of the front wheel hub. Were older VW's similar?
Update: I have checked my old Greg Schmidt parts book on 600/850's and he does not mention any parts available for speedometer conversion.
I am contemplating an OT2000 conversion/tribute myself with a Lancia Beta block, and was wondering if a Porsche Type 901 gearbox would fit without sheetmetal modifications. I believe that 901 gearboxes have a speedometer drive on them:
http://www.californiamotorsports.net/901.htm [nofollow]I have read that the 901 gearbox is a bit smaller than the VW box.
There are many shops on the web that advertise recalibration of the speedometer.
This shop may do it for Veglia instruments
http://www.paspeedo.com/ [nofollow]Some Type 915's have an pulse electric speedometer, I have no idea what it would take to adapt something like that...but then I have heard that the gearbox is too big to fit into an 850.