IMR4320 was preferred by Ken Waters when he did his writup on it with a factory chambered Ruger M77 (Pet Loads). They didn't follow through with that round in production, but should have.
; Yes to using .338Mag. brass - necking up .338's is a very good way of making brass. I find it also better when making my .458 2" as well. Depending on your die mfg'r, you may not have to go to a different necking plug. Hornady will make the dies and their normal plugs are nicely tapered.
; If you want some data, mail me & I'll look itup for you.