From what I have read they were offerd in 1960's Med duty trucks. It is a popular retro for LD trucks as it hs a low forst gear, direct drive high gear. Better overall than say a mucie 4 sp. Splits are supposed to be better also for on road conditions.
Clark 5 speeds were put in everything under the sun. We have a dozen junkers with clarks in them, chevs , fords, macks, reos. They all have secondary boxes,and worked good according to my grandfather. Performance gear specializes in clark swaps.