Hopefully you have a Saginaw in that '66. The Saginaw has a 7 bolt side cover. There was also an optional heavy-duty Warner 3 speed, used from '63-'67 and maybe more years, not sure. It too had a non-syncro 1st gear like the SM318, but it should be a top-loader with a 9 bolt cover.
TruckNut
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