Well maybe I'll start with a more detailed picture of my steering shaft.
You can see in the picture the shaft is .750" .120 wall CroMo and the ends are the Woodward 20 spline style. This style is pretty cool with a square edge on the top of each spline. This prevents the spines from splaying wach other apart. You can see in the photo below the difference between the Woodward spline and the std automotive 36 spline that is found on my rack and that I ordered for the end of my Flaming River colomn.
The other day I assembled my rotors. I still have to safety wire them but that is another post. Here is a picture of one of the rears. The hat is cast and acts as a brake drum for the parking brake. Yes full race junkys I plan on running a parking brake. The rotors are 14x1.10" e-coated and slotted but not drilled.
Today I removed the hubs from my Speedway floater and stared installing the brakes. This is just a test fit install.
The powdercoaters coated the flanges for the brake backing plates. I started by chasing the threads with a 3/8 24 tap to remove the PC
Next I took a flat file to the mounting face. I would prefer this to have no coating so the backing plate would mate to a machined metal surface. For not I just spot faced it to ensure it was flat. I may strip it entirely. More on that later.
On with the backing plate and parking brake assembly.
Now here is what I'm talking about with the powdercoat. The Wilwood caliper bracket mounts to the back of the backing plate. When you assemble it you use shims to center it on the caliper. They say to start with two .11 shims. You can see them in the picture.
Then there is another spacer between the bracket and backing plate.

The radial mounted caliper bolts onto the bracket. You adjust the centering of the caliper with the shims. I would guess the powdercoat is already spacing the caliper .11" or more outboard so although I stared with 2 shims as Wilwood recommends I really only have 1 shim of spacing. If I need to move the caliper inboard more I'd need more shims than normal. If I need more than three I'm going to clean off the mounting face. The Powdercoat is like a hard gasket of sorts and the force on the bracket isn't going to work on the "gasket" so I'm not overly concerned about it but I may be cleaning it all off the mounting face.