The drive-gear is on the transmission shaft, and the driven-gear screws into the transmission (with the speedometer cable going into the end of the driven-gear piece).
These gears are matched at the factory to the tires and to the rear end.
The probability of finding a matched-set of 1/2 ton gears in an SM-420 is low. If that transmission was in any other sized truck (or, was in a 1/2 ton with different diameter tires than on the present truck), the gears will not result in a good speedometer reading.
My suggestion is to find/use any set of drive/driven gears for an SM420 with enclosed drive-shaft-end. Then, do the measurement to order a ratio converter (
The Speedometer Shop is one place that makes these). I have done this twice - it provides spot-on mileage (w/3.55 rear end and/or taller diameter radial tires).
