since you're just updating the stock brake system, it can get a wee bit easier if your ball joints are in good shape.

Take your new pin type caliper adapters, and the late B-rotor.

Don't swap spindles, buy the bearing spacer from dr. diff. Now you don't need to pop the upper ball joint and the swap can be done in the time it takes to do a brake job. The only caveat is that the 71 dust shield won't work with the late B brakes. (this would be true even if you swapped spindles.

This is what I did on my 71 challenger and put hundreds of trouble free miles and several dozen autocross laps with no issue.