It will work fine!!! I ride the same setup with steel frame and pressed sheet metal bearing holders. Never had any issue.
You just need a little adaptor piece or some filed shims.
See his thread where we discussed this issue. You need a planar contact face so that the brake force can go into the frame and not destroy the bearing. And this will also prevent you d’brake from cracking early.
See also my build with the adaptor (bad translation via google). I can also provide technical drawings and CAD data for the adaptor in nearly every CAD exchange file formate.
If you want it simpler, refer to Hugo’s build, he used shims instead of the adaptor and it works fine. (bad translation via google )