My four door has the original power drum brakes with a single pot master cylinder. I would think that front disc set up would be a little better and eliminate any of the side to side pull associated with out of adjustment drum brakes. Make sure all of your disc components are matched to each...
I don't know about others, but the drum brakes on my four door work great. I can't imagine needing more than front disc with rear drums, but everyone is different in their preferences.
That’s the dome the nut is under. It’s a caged nut and will probably take some cutting to get it out and replaced. Here is a picture of what it looks like inside the boxed section.