A body - B body spindle swap

Any of the 73+ F/M/J/B/R body spindles will work. They are all the same. The B/R cars had the option for the larger brakes, so they might have the 11.75" caliper brackets. All of the F/M/J cars would have the 10.98" rotors and smaller brackets.

The F/M/J/B/R spindles not working/causing problems is complete BUNK. I run them on all of my cars. They're fine. If Ehrenberg had actually bothered to check the geometry instead of just saying they're different and therefore won't work, this wouldn't be a problem. You can see the actual geometry analyzed here. Real numbers, actual geometry.

http://www.hotrod.com/how-to/chassis-suspension/mopp-0503-swapping-a-and-b-disc-brake-spindles/

If anything, running the later spindles is BETTER for cars with wider, modern compound tires.

As for the master cylinder- manual disk A-bodies ran 1-1/32" bore m/c's, power disk A-body's ran 15/16". Using a 15/16" bore m/c with a manual disk set up will result in an increased pedal effort, but also a higher line pressure. The 1-1/32" bore with manual disks will have a softer pedal effort, so it will be easier to push, but the line pressure generated is lower too.

I run my Challenger with F/M/J spindles and 11.75" B/R rotors with a stock power m/c (for now), and my Duster with F/M/J spindles and Dr. Diff's 13" cobra kit with a lightweight 15/16" manual m/c, also from Dr. Diff. The EL5 GT is getting set up with F/M/J's and 11.75 rotors as well. It will probably also get a 15/16" bore manual m/c.

Also, you CAN use the earlier, 70-72 B/E-body disk spindles. Dr. Diff makes a sleeve that allows you to run the later inner wheel bearings. Not my first choice, but it can be done...

http://www.doctordiff.com/70-72-b-e-body-disc-brake-spindle-sleeves.html