You can add a suregrip to your 741, they are acutually pretty easy its the pinion depth that is somewhat more difficult but you won't be messing with that just changing carrier/suregrip a dial indicator is all that's needed and a tube of red locktight.
The backing plates and axle tube end bolt pattern is different before a certain date, I think it was the same year as ma-mopar went with the Bendix brake style as well. I didn't know about the taper change though
If you change the entire chunk keep in mind that your tapered axles don't have bearing preload adjusters like the newer 8.75 axles and use shims to set the bearing preload. I'm mentioning this because the early chunks use a different (square) thrust button in the centre and the clutch style suregrip uses a 2 pc button with a dowel and the newer open and Cone style suregrip use a one piece floating thrust button. The point being you will have to make sure you shim correctly for the possible change as I do believe the old square thrust button is thicker than the newer styles and therefore more or thicker shims will be required.
I agree with others mentioning that if you not restoring to original that a complete change to the newer 64+ 8.75 rear assembly.