Okay, figgered it out. Started with a fresh 416 rigby ejector and stoned it until it worked exactly like the 458 (modified) that came with the rifle.

Turns out the real problem is not the rim DIAMETER but the rim THICKNESS.

There appears to be no factory chambering in a No. 1 where the rim is a thick as the 450 Nitro #2 3 & 1/2". So the Ruger is set up to push the ejector far enough forward to engage any factory chambered rimless or rimmed cartridge. The ejector cut in the barrel is deep enough, as after the roller pushes the ejector up next to the rim (as far as it can) you can push ejector "home" with a skinny punch.

Therefore the REAL problem is the front to rear dimension of the ejector arm, as the roller cannot push it far enough forward.

If I were home with all my tools, I would drill and tap the roller side of the arm and put in a small allen screw. I'd adjust that until roller seated the ejector "home" and red locktite it in place.

For now, I have backed the ejector tension screw all the way out, so that the load/unload is like a traditional Winchester High Wall. You push the cartridge in with your thumb and it pushes the ejector (which is now just an extractor) home. When you drop the block, the "extractor" pulls the case out about 3/4" which allows you to dump it or hook it out with a fingernail.

Welcome back to 1885 !