Problem solved: I called up Hugh Henriksen, who has made a number of reamers for me in the past. I was reluctant to ask Hugh, because he is generally covered up with orders, but as it turned out, he had a roughed out 8X64S reamer in stock, and because it had not been finish ground, he will be able to grind the neck to the same dimensions as that of the 8X57IS (9.08mm/.357"), AND he will be able to get it to me in a couple of weeks!

Incidentally, I can recommend Hugh unreservedly. His work is right up there with that of the late, great "Red" Elliot.