I'd contact POL - Performance Online - via phone - and tell them what you are looking for. Not sure they have it, but they are honest and they have a lot of disk conversion parts available.
https://www.performanceonline.com/Br...evy-GMC-Truck/
Not sure about the zinc coating - does the truck sit a lot? That will keep rust off the surface. Might consider painting the non-brake surface of a non-coated one before install to preserve. Not sure this helps! This thread and forum is close enough - assume you've done a conversion on your 63? Many upgraded 60-66 to 71+ spindles. Ask about the drilling and slots - I'm pretty sure they have those available or can do it to something you want to order.