The Clarks of that era are all very similar. Hollander of the era lists what is interchangeable on civilian, but does not list military. But Clark did not change more than they had to to meet the military's specs. I would guess that a civilian back section would fit, but its not going to be that easy to find a similar Clark. If you find the back section, you will probably have to buy the whole transmission. If a speedo is that important to you, I would be cautious in the matter. If you can get into it for scrap iron price, it would be worth trying. But I would be hesitant to pay much for it with planning on getting a speedo to work. For me personally it would be fine because I don't care if a speedo works or not.
