Hey Superman,
The drilling of the speedo hole might be very difficult. Have you ever let it do by a shop?
To overcome the problem I've used a bike (yes, bike) aftermarket digital computer speedo: they work with a magnet to the bike wheel and a pick-up point on the frame (same principle as a dizzy hall-geber or uni-lite system). So I attached the magnet to the 944T disc inner side and the pick-up to the spindel. The digital speedo has a glue strip on it that I sticked to my tach, which is in the middle of my 03 dash instead of the original speedo. You simply measure the radius of the front tire and feed it to the bike 'computer' It has sevral functions like highest speed ever reached, average speed etc.
I now have digital accuracy :-) and can dispose of the original speedo, which I couldn't use any more anyway.
BTW highest speed has been 188.7 km/h... which was 5950 rpm with a 3,875 stock trans
Walter