I've made 35 Win rifles on the #1 SMLE with complete success. Magazine is the problem with any SMLE conversion. It's quite tapered back to front to accomodate the 303 rim at the base and pointy 303 bullet at the front. The 35 Win has to have bullets seated deeper than for a M95 Win, but still work well. As bullet diameter increases, problems compound. Don't know if .375 will work, but 405 does not without major changes. I'm convinced that straight walled cartridges require a single stack mod to the magazine for reliability. Would be interested in details about such mods from anyone who's done it.
Otto