you are absolutely correct. The 6.40 truck gear was in trucks that were used in transportation in the mountains. I'd take a complete rearend from one of those trucks. Either that or a 6.37 gear. I am not sure what you mean by sizes. Gonna have to educate me on that one. All im telling you is what my grandfather and his friends are telling me. I'm sure THEY know what they are talking about, but its hard for me to convey it if I dont know. They are talking about original gears that would have been in a Ford/Merc from the 1939ish era or the 1940's. Does this help?