A good reason to run braided lines on rear disk. Ouch!

I talked to Cass and he recommended trying to swap the adapter brackets side to side so that the calipers are on the fwd side of the axle instead of the back. If that doesn't work, I'll have to weld and re-drill the holes in the bracket or I'll be notching the frame. This isn't his first time to see this at least and said that those were essentially my options at this point.

Well at least Cass had some options for you, even if they might not work. Looks like swapping them side to side so they're on the front side of the axle would put them closer to the highest part of the arch in the frame, that might buy you enough clearance.

Did he mention what the minimum width of the axle has to be in order to avoid interference like this?