Progress.

I had some seconds of the 8x58r360 brass and they do in fact fit. I ran the case through the 35 whelen die to open it up and I used a 9.3x72r expander. Surprisingly it worked out.

The case fit in the chamber with a .366 speer, the outside of the neck measuring .392. Just to see how much room there was around the neck, I added one thickness of celophane tape to the outside of it (.396). There was no way that it was going to close with even that amount so I think that I am in good shape.

Now to find some lead...

Tom