Why can't you just swap out the spindles? If they had disc out of the factory for 2500's then shouldnt the spindle be the only difference. The five lugs were that way and real simple to go from 5 drum to 5 disc. Anything stronger in 67 (i.e. 6 or 8 lug) would take finding the disc spindle from a later year. Unfortunatly the 6 lug disc didn't exist until later (correct me if I'm wrong on that) when the frame changed, however, I am currently looking into the fitment of a 6 lug from a 1988 heavy duty application to convert my 67 GMC drum. As for rear disc, go for the whole axle late model. Your pocket will thank you later (and your Brain) and a serious upgrade on a whole. ^ BANG for $$$. Don't forget the master cylinder...
