Hoppdoc is correct. The old tang safety rifles are not CRF. They are pushfeeders and must be to allow the cartridge case to seat ahead of the plunger ejector.

As for the .338 vs. the .458, my vote goes for keeping the .338 also. Here in Idaho that .338 is one of the most useful cartridges ever designed. Obviously the .458 is the one to choose for certain limited DG uses, but the .338 is arguably one of the most versatile cartridges for all North American game and for 90% of what is shot in Africa also.

You have a real winner in the .338.