Hello- I've got one of the these too. It is called by its nickname, a 3 speed "Brownie", used as a second transmission behind a 5 speed Clark, or whatever trans is in the front. It provided an underdrive for pulling, drive for one-to-one, and an overdrive for cruising. Got mine from behind an older Cummins Diesel, but they are useable about anywhere. Was thinking of putting mine behind a c-6 or a TH400 to make use of the overdrive, but really its original use was to make a fifteen speed trans out of a 5 and the Brownie 3 speed in 1950's semis.... They are really heavy duty, and have provisions to bolt on a PTO too, in case you want to run something...compressor, generator, hydraulic pump, etc. Haven't seen many, but i don't frequent the older big truck wrecking yards.
If you can come up with a shifter to get the three rods to move correctly, the rest should be easy. I haven't seen what the original shifter looked like, but maybe that'll give me a reason to start looking. The mounts on mine look like 1" thick by 2" wide bar-stock...pretty heavy for a pickup. The guy I got mine from thought they went for about $600 and up.... I traded for mine and thats about what I have into mine. Curious what you are putting yours into... Oh...ratios are 20% underdrive- then 1 to1- then 20% overdrive, if memory serves me correctly...
Steve