JPK
As I have stated in a previous post I have handled your rifle and it is very nice. JJ is correct the 450 3 1/4" would be the correct ctg to rechamber to. I would try New guys technique as well as using a little "thumb" action on the base of the cartridge as I pushed it into the chamber. It may just be a question of finding the right technique.
If you fine a correct technique you might even like keeping it as a 458 Win.

As regards to the 350 grain bullets I have had excellent results on my 450 No2 with 350 gr. and with 300 gr. in my 450/400. The key is to use the exact same powder charge that regulates with the heavy bullets.
I would think that in your 458 that with the 350 grain Hornady RN that somewhere around 70 grains of IMR 3031 should work.
I would start with 68 grains of IMR 3031 and work up one grain at a time.