Welcome to the board!
There is alot of info here, on doing the disc brake conversion:
http://67-72chevytrucks.com/vboard/s...68&postcount=4
I would look for a complete 71-72 rearend.
They should be a bolt in swap.
What most people don't realize is,... the front track width on the trucks has always been wider from the factory.
Look at your first picture.
It's easy to see, that the front tires stick outside the fenders more than the rear tires.
Adding the disc brakes makes it worse.
I like to use different width wheels to help correct this.
(either 8's and 10's or 7's and 8's)