<<I can switch tail shafts provided I change the speedo gears, right? Or as you mentioned alter the shift rod.>>
If you want to change the tailHOUSING, you need to change the output SHAFT, or you won't have the machined surface in the right location to accept the pressed-on speedo drive gear. It would be simpler just to modify the 1-2 shift rod so it clears the speedo cable.