Big bolt pattern and disk brake conversion

Professor Fate said
YES THEY ARE DIFFERENT! As stated, and as outlined in his link.
Different upper control arm, different spindle/knuckle, obviously different caliper and adapter and rotor, different master cylinder, different proportioning/distribution valve, different line routing... Different!

No, the UCA is not different on the '73 up cars. At least not past November of 1972. It's in the TSB. The drum brakes were swapped over to a B body based unit with the modification to the hub for 5x4". So the spindle is different from a disk car, but the UCA uses a large ball joint just like the 73+ disk spindles.

https://www.hamtramck-historical.com/images/TSBs/1973/05-03-73C page1.jpg

https://www.hamtramck-historical.com/images/TSBs/1973/05-03-73C page2.jpg

DionR said
Here is an UCA I know came off my ‘74 Duster with disk brake spindles. The spindle is a ‘73 drum brake spindle off my '73 Duster project car. Fits together well, UBJ is the same.

View attachment 1716277481

Funny thing is, I have the drum brake spindle because we converted the ‘73 to disks back when the car was my son’s. And I am almost certain we didn’t change the UCA’s when we did it. I know I had to do a LBJ, but only because I mushroomed the stub. So even the LBJ should be the same disk to drum.

So now you have seen a big ball joint A-Body drum spindle.

This. '73 up drum cars were SBP and LBJ.