IT WORKS !

I dropped one of the 45-70 ejectors into the 450 for a trial fit to see what fitting might be required.
Dropped in a case and raised the lever expecting it to hang up somehwhere on the 450 rim. Nope closed right up with the ejector under the rim. Dropped the lever and it extracted the empty.
So I tightened up the ejector adjustment screw and tried again. A bit more effort to close but chambered with a positive snap. Dropped the hammer, then the lever a ka-ping, the empty was ejected far enough to be clear of the barrel, sitting in the trough. Easy to dump out were a 2nd shot be needed. I could set the ejector spring even tighter to have it clear the gun, but that's not necessary for range shooting and will, of course, make chambering take more effort.
Just scored another 4X wood No.1 in 458 WinMag. Fiddleback end to end ! It's a "black pad" and has the typical open grain from cheap sprayed on finish. That can all be fixed. Now if I could find an original red buttplate, that would be the final touch.:)