Transmission is in my 1966 Convertible, but I would not mind upgrading to an overdrive transmission. It is a 4 speed M21 (coarse spline, one(?) rings) or M20 transmission from a 1967 Corvette - serial # 7S113708 is stamped on the case. I really expect that the donor Corvette was junked in the early 70's, as that is how long this transmission has been in my car. The transmission was rebuilt a few thousand miles ago, so it should be in very good shape.
You can see the input shaft here and guess for yourself if it is an M20 or M21:
If you are interested, I will go under the car and get more info (and take pix). If you are serious, I can pull it (I likely have to soon to install the engine anyhow) and take interior pix.