Whats needed

• Upper control arm from a 73 or later A-body. Drum or disk brake car can be a donor.
• Disk brake spindle & accoutrements from a 73 or later A-body. Supposedly the same from a 69-71 B-body will work but it changes the suspension geometry a little. I may have B-body years wrong. Other readers, feel free to correct.
• Disk brake master cylinder, junction block/proportioning valve (can be 1 piece or 2.) front brake hoses.

At the risk of being guilty of stating the obvious, this will leave you with a 4½-inch pattern on the front and a 4-inch on the rear. The simplest solution is a rear axle from a 73 and later disk brake A-body. These will be either 7¼ and 8¼-inch units. The drum brake A-bodies all used the smaller pattern. There are several other solutions out there, but I'd be going beyond the scope of the question to even attempt to provide a list.