Minor corrections. I think your "billions of oz's of physical silver" is a bit of hyperbole. That said, the reason why silver was used in the calutron was because it had originally been wound with copper which was extremely important to the war effort. They literally had to take the damn thing apart, rewind it with silver wire instead of copper and fire it back up. After the war the process was reversed.
Interestingly, the engineers could not get the machine to work the way they needed it to. They were close to a redesign when someone decided to let women operate the controls. The women had no problem maintaining the parameters needed to produce a positive result.