I'm also concerned about only using two mounting holes. The original backing plate used 4 and you now have more brake capacity with half the support. I'd feel better with three, four if possible.

Is there any room to cant the caliper forward or back any to assist with bleeding, you may never get air out of the other piston? Ideally the bleeder is the highest point.

Looks like steel to me and it should be, I wouldn't trust aluminum either.

Larger studs are a good idea, no reason to use bushings on small ones, just replace with the larger 9/16" studs.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-