Stock speedo gear?

Here's the fix without tearing up your trans...
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy with within 1%-3%. The drawback is that it isn't cheap, I have heard $75-$100 for the kit & speedo calibration.

Check the site out for different combinations. There is almost always a combo that will work to correct the speedo -however, sometimes replacing the drive gear is necessary, and that requires getting to the tranny output shaft.

http://www.geocities.com/mgnedell/notchback/tech/speedo.html