Roger, could you clarify which rifle you will use to build this blaster? Classic Safari Express???
In my own opinion, all of these hogged out 458's will have problems. Excepting the 458 Lott, of course. If someone accidentally loads the orig. cartridge (458 Win Mag.), the CRF claw extractor may hold it up to the firing pin.
If you design one like mine, and deliberately chamber it from a new short chambered barrel,(-.050"), then you can leave it about .025" shorter than a factory 458 Win Mag case. I'm hoping that's enough to preclude a Winnie Mag from chambering, in the larger diameter chamber.
But I'm finding out that big bulbous bullet noses and cases larger than the H&H belted ones, mess up a Mauser M-98's staggered feeding. My 10.6mm x 375 Ruger wildcat rifle is up at the G.S. right now, for some extra machining under it's left feed rail. The 410 Woodleighs, are popping up through the left feed rail. This is partly due to the extreme taper in my cases. It matches the Schulers, and Schuler did go to a single column magazine for their 8 x 68 S Magnum.
These rifles must be absolutely reliable. So, I may have to hit Dave Kiff, at P.T. & G. up for a new finish reamer, with less taper. Dave said this would be a possible cure for a lack of shouder to head space on, but now it looks like it may cure a feeding problem with the 410 gr. RN Woodleighs. A Rem 7mm Mag feeds just fine, from both rails.
This problem arises from the front of the Mauser staggered feed system, deflecting the bullet, nose first, into line with the chamber. The added taper in the case body, means that the stock rail can't hold on to the case heads long enough. Ergo, Schuler's single column solution.
Only the Schulers and the 458 American have as much taper as my Boer 8mm Mag. and 10.6mm x 375 Ruger wildcats. So I suggest you work some dummies up and check your feeding before you re-chamber your existing 458 Win Mag.
My G.S. says he can retro fit a Weatherby Mark V mag box into my Rem LH M-700 RUM rifle, for really large diameter parent cases, like the 378 Wea., but this tuning will be expensive. And my Remmie isn't a CRF action, anyway. If you have the full length CRF action, then maybe a 458 Lott or a 416 Rem Mag. would be the way to go. If it's a std. length one, then there's still the 416 Ruger.
BTW, I use the 375 Ruger as my parent case, and both my shoulders run .483", just like your necks. I need absolutely minimum case taper for my .450 Magnum, and they can only be made from the Ruger Basic case. As my .450 has less taper than the 375/416 Rugers. In a cheap M-98 Mauser action, it may have to be built as a single column feed job, from the start.
If I get the feeding problem solved, then I'll work over my Rem. 700 L.H. into a .500 caliber short. Not as any back up, but as a quicker shooting, belted rimless 500 Alaskan. My Mauser was really too slow to reload and shoot Kudu, left handed, back in May. So, will you keep this forum up to date on your project? If it works out, you will be a step ahead of me.
