Bill,
Jack Lott talked about using .404 brass & sizing to .425 Westley Richards, but using an opened out bolt to suit the larger rim - this would retain easy feeding.
He also mentioned having a second bolt, so if required he could use the proper rebated cases when available. This being said it depends on what you really want to use it for, the .425's were in most cases basically single feed (correct me if I'm wrong here fellas) so it will be a real labour of love to get this one up & running properly if using correct brass!
Every thought of a 10.75x68mm, it can push things along if required & there are plenty of 'classic' rifles in this calibre - load data should be here just use the search function.
Best of luck with the project
93x64mm