If not wanting to go too big the 375Flanged Mag would be perfect and you can probably readily buy the excellent Norma brass from next door!

The 450NE could be nice too, depending what the expected weight and balance of the rifle would be.

My 500/450 3 1/4" Mag ( made in 1906) came with a set of 375H&H Flanged Mag barrels by H&H (made early-mid '50s), perfect. But it does make for a heavier 375 than I might prefer. Whereas going from 300H&H to 375H&H may be just what the doctor ordered!

Cheers,

Chris