My first question would be... why did the P/O put a wood plug there?
I would call Nevada Speedometer service. They do rebuilding and calibration of speedos. You should have an early sm420 (MUNCIE) transmission in there. Most 3/4 tons came with 4.57 gears, and most 1 tons came with 5.14 gears. Does the torque tube go all the way from the transmission to the rear axle? How many lugnuts on the wheels? it sounds like you may have a 1/2 ton with 4.11 gears by your description. Verify your gears, and take a measurement of the distance traveled by one complete rolling tire revolution.
John
