I think you need to find a correct pair of "drive" and "driven" gears for the transmission. Good luck.
Or, it costs about $100 to have an adapter made. I did this after changing to a 3.55 rear-end and changing to taller radial tires - it is now within 1/2% accurate at 60 mph.
Speedometer Shop I ran 4 measured ten-mile segments on an interstate at 60 mph. Sent them the average of the four measurements.
