Help converting my 64 to 66 23 Spline 8A33 to an overdrive ?

You must use a O/D mainshaft. The way I'd change your 64 to 67 A-833 to an O/D is to use your main housing and buy an O/D gear set, a 26 spline O/D main shaft, and a 26 spline A-Body tailshaft housing, and a 3-4 O/D shifter rod. That way it will just bolt in to what you have now. Assuming you have either a B&T or small, 26 spline output mainshaft.