I spent a lot of time looking into clark transmissions a few years back. I even wrote up my findings for this website, although I don't know if it is still around. I had talked with a shop that specialized in the trans, and typed up what he said. I'm doing this all from memory, so double check the information.
The overdrive versions were usually in delivery trucks or over the road trucks, not farm trucks. The overdrive wasn't all that common, but it wasn't rare either.
The overdrive ratio was .80 (20 percent overdrive), no other overdrive ratio was available.
The tag on the overdrive transmission will usually have a VO or O on the end of the tag number, except for later versions. Later versions have to have the tag number deciphered to know if it was an overdrive.
Later versions are significantly larger than the SM465 transmission, which came in most 70s pickups. I doubt that they would fit under the transmission humps of most pickups from 47 to present, but would be OK in 2 ton and up trucks.
Earlier versions were about the same size as the SM465, but some earlier transmissions were not synchronized, so be warned (double clutch it).
