Hello, I ended up going with Possum Hollow for the bore guide. They will not be able to install an o-ring but will have a deep enough throat that it shouldn't be a problem. PMA was very responsive and honest. Please see their response to me below.
Thank you for your interest in PMA Tool.
The 458 Win Mag is pretty much a straight-walled case, therefore an o-ring snout isn't able to seal in the chamber while still leaving enough room inside the rod guide for a jag with patch or a brush to pass through. We can make you a custom rod guide for that rifle, but it will have a tapered snout in place of an o-ring. In reality this is no better than using a less expensive rod guide from Boretech or Pro-Shot.
I'm happy to make one for you, I just always want to make sure we educate our customers, especially when a custom item, may not do the best job.
