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.
thanks for the confirmation on that. i *thought* the OD shaft was different but didn't have one sitting right here to compare and contrast. i figured the rest was just a: match the tail to the shaft and get the gears.
you lose the big input bearing, but you can use the existing bell which is nice. ostensibly, you keep what, the case, bearing retainer and counter shaft?
anyway, to me that seems like a lot of work and expense for something that's available and floating around out there; or... spend a little more and drop in a 5spd out of something and get better ratios and parts availability.