As long as the bellhousing bolt pattern and the input and output splines match up, and the overall length is the same, they should swap. I think there might have been more than one ratio selection on the 1st. and 2nd gears. 3rd. will always be direct drive, a 1:1 ratio. Do the cases look the same? By 65, I think all the 3 speeds were the fully-synchronized Saginaw units.
Jerry

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