Thanks for the two replies so far. I have heard of the 458 based on the 300 ultra case.
But as I said in my original post, I would like to hear from people who have had actual experience loading and shooting similar cases and can comment on any issues they have had regarding headspace. This is the only issue that is holding me up from proceeding with this build.
I know there are numerous other 458 cartridges such as the Lott etc, all of which may do velocity wise as much as the cartridge I'd like to build. But as all of you wildcatters and experimenters know, very rarely is it only about velocity. A lot of times it is more about the fun of the design and build process, and having something that very few other people have when you're done.
As far as the 458 American 2", that's going the wrong way in velocity for me. If I was interested in sub 458 win mag velocity, I would just buy a 458 winnie and load it down.