rebuilt transmission that has been sitting

SIX225 is correct about the splines on the input shaft (late 1962 to 1967): crank pilot 1.550",18 spline.
1968 up, crank pilot 1.810", 27 spline.
So depending on what engine it's going behind, it may or may not be such a good deal after-all. The converter might be an issue, not one that can't be corrected with a custom build converter, but a possible issue that you need to look at very carefully.