setting up for disc brakes will not change anything on the rear ends width. tha axles are a set item and do not move. if you are looking at new gears and disc kit, look at about 1000 to 1200 bucks extra on top if the 900 for rear end cost. 9" over 12 bolt if you have a choice, but the 12 bolt is fine if you already have one. as far as gear changes being easier, yes much easier with a 9". but once you build the car and select proper 1/8 mile gear. its not like you will be changing gears everytime you hit the track.