The exact same thing happend to me when I converted to a C4.  I took the car to United Speedometer in Riverside, CA  (I lived there at the time).  They made a gear box that mounted to the back of the speedometer head to get the proper (well almost) reading.  It actually screws to the back of the speedometer head using a fitting like that on the end of the speedo cable and the speedo cable plugs into the other end.  As I recall it was a bit noisy.  I was looking for something the other day and happend across it.  Interestingly, when I switched the car to a 5.0L with AOD the speedometer read pretty acurately with out the gear box.  Give me a call at 402 461 6080 and we can visit about giving this a try.