No, the granada brackets/calipers will not fit on the pinto spindles. You will need custom brakets for any calipers you use with the granada rotors on a pinto, but the brakets for GM metric ones are out there (and cheap). I have not heard if the granada spindles will work on the pinto a arms, but I tend to doubt it. Even if they bolt in, the geometry might get messed-up.
I have not done it, but you will need to do some cutting and welding to install rear disc brakes on a pinto rear, or to install a rear with discs in a pinto. There is a lot to it, and I would say that drums are easier in the long run. If you plan to do it try to find a rear that someone has converted and is ready to bolt in (I have never seen one for sale). If you can't, and you want to make your own, get a full donor rear with ebrake cables so you will have all the parts you need. I installed a rear from a 68 stang and it took some work, but the pinto ebrake cables worked fine. This spring I plan to re-do the shoes on mine, the rear has big drums and I get too much braking from them. I am going to use 4 'front' shoes, or install a manual proportioning valve.
Bill