9" drum to disc conversion confusion
Just get the willwood kit and be done with it. 659 bucks and all is new and you can go big or small bolt pattern. Never understood why someone would spend time and money trying to mix and match a bunch of used parts when you can get an easy to install kit that works great.
Hmm, lets see...
FMJ spindles, backing plates, caliper brackets: <$100 at local yard (yes, I've bought them for less than $100. Three times now.)
rebuilt calipers: $62 @ Napa
new semi-metallic pads: $15 @ Napa
new rotors: $100 @ Napa
new bearings and seals: $20 @ Napa
new ball joint adaptor: $50 @ Dr. Diff's.
Total: $347
That's pretty much like for like, seeing as how the wilwood kit doesn't include the brake lines, master cylinder, combination valve, etc. And the wilwood brake lines will cost you more than a set of 73+ stockers. Plus, if you're at the yard pulling spindles, you can get the combo valve too. As far as "mixing and matching" old parts, you don't have to. The only used parts you really need are the spindles. Combo valve is a bonus, but you can buy them new also. Everything else I just buy new because the prices aren't much worse than the yard when you start considering core charges and the condition of some of the wearing parts. And the only thing that's being "mixed" is the spindle to the UCA. The rest is all intended to work together, all bone stock.
If you do go with the wilwood kit, you're stuck with them for any replacement parts you'll need later as well. With the 73+ Mopar stuff you can just go to the local parts store, RockAuto, whatever. The 73+ disk brakes were used on a lot of cars well into the eighties, and on the 5th Aves and Diplomats all the way up to '89, so those parts should be available for a long time yet.
As far as the FMJ spindle geometry, it is almost negligibly different. It shouldn't effect your alignment geometry enough to need the offset UCA bushings, although I recommend those anyway to improve on the stock alignment specs, which were for bias ply's. I'm running FMJ spindles on my Challenger and my Duster, I've had no issues with either one.