3/4 ton drum to disc for '69 rear end

[69CamperSpecial]

Ok from what I have read on here looks like rear discs became standard at some point? What year would that be (if that is correct)?

The plan is swap entire rear-end if I can find one in a junk yard - only other question is are the rear-end the same width - seems like I read that the axles got longer in the 71-72 time frame - assume an inch on either end isn't an issue if I replace the whole thing...

Any issues with drive shaft legth or u-joint size I need to think about?

sorry for the newb questions.

thanks
Ian