I second the 416 Ruger as the number one choice, its a given almost...
Using Atkinson math and therory, necking down a 404 case to .416 would be no problem at all, that would be only seven thousands and divided by 4 sides, that ain't much.
I built a 416/400-3" (a 450-400 necked down to .416) double rifle once, still have the dies, not much market for those dies as they have not sold in about 50 plus years! back then it was superlative idea as .416 bullets were available and .423s were rather hard to come by..The guy that bought the rifle wouldn't buy the dies if you can believe that, maybe he wanted a wall hanger!
But the 416 Ruger would be, by far, the best route with brass and ammo readily available, conversion much simplier, and balistics would be the same I suspect, at least close enough that a buffalo couldn't tell the difference and you have the advantage of a proper headstamp (if thats an advantage) plus the availability of factory fodder...
