AA2230 is the best powder for 450 to 500 grain bullets in the .458. You can get 2200 or 2250 fps with no excess pressure. H335 is also good. For light (350 grain) bullets use 4198 and you can get 2700 fps with no problems.
The only problem is that these loads cause the 350 and 450 grain bullets to shoot about 7" different in elevation at 100 yards.
My rifle has a 22" barrel. It has never got "hung up" when hunting elephant or buffalo in the African jesse. Try that before cutting further.
