Yeah, the .338 neck up seems to complicated as you have described with multiple sizing dies needed. I'll just buy more .458 brass if I decide to build a .416 Taylor. I'll ask a gunsmith acquaintance about it. He builds custom rifles mainly with the .340 Weatherby mag cartridge. Thanks!Necking 458 down is a good bit easier than necking 338 up IMO.. but either one works..
Necking up from 338 is a 2 or 3 step process from what I can tell.. whereas necking down from 458 is a single pull of the lever at the reloading bench.. run 458 brass up a 416 taylor full length sizing die and you are done..
Most people I have talked to that use 338, first neck up to 35.. then neck up to 375 then take the neck to 416.. going straight from 338 to 416 appears to be an issue (I have never necked up from 338 myself.. so I am strictly going off of what I have been told by a couple of other 416T owners and what I have read on some other forums that are more reloading focused)..