There are three models of Warn/Saturn overdrive:
1. There is the version for the early 26-tooth Model 18 transfer cases. These were used in early post WWII jeeps, were all small hole
cases and had either 3/4" or 1 1/8" intermediate shafts.
2. There is the version for the later 29-tooth Model 18 transfer case when connected to a T-90, T-86 or T-98 transmission with a 6-spline output
output shaft. This could have been a big-hole or a small hole transfer case dependent upon which transmission was bolted up. They
all had 1 1/4" intermediate shafts.
3. There is the version for the later 29-tooth Model 18 transfer case when connected to a T-14 transmission with a 10-spline output shaft.
These were all big-hole cases with 1 1/4" intermediate shafts.
If you used the original Model 18 transfer case from the GPW in your buildup, it has a 27-tooth input gear and there is no overdrive
available for it. It also has a 3/4" intermediate shaft. I suspect your SM-420 has a 6-spline output shaft.
Reasonable price varies by location and condition, $400-$600 seems pretty typical. (they're ~$1200 new)
1946 Willys CJ-2A