+1, I agree that ALL big blocks got the short shaft gear box but there is an early AND late version of the short shaft gear box and I would say your build date would get the later one.
Without doing the research again, (you should continue to look for the "when" the change-over took place) I can add to the discussion there were two different short INPUT shaft gear boxes used in '67, identified by the OUTPUT shaft diameters (early & late)... and subsequently, two different pitman arms used. (Early version=small diameter output shaft, Late version= large diameter output shaft). Jim was correct, I believe some clues would most likely be in the MPC to narrow down the date when the change occurred. There is also just as much likelihood the change-over is just an estimated date.