Well, lessee, old-catalog-correct choices would be the aforementioned .375 2-1/2" Flanged and the ever-popular 8x50R Mannlicher. With that out of the way should you want to try a 7mm on the std .303 case I have a once-used reamer I'd be happy to give you along with a stack of WW 303 brass, maybe even throw in a '92 Mannlicher sporter redone to 7-303 as well.
Other odder (!) choices might be one of the x57R rounds, most of which have modern available brass these days though the factory ammo might be too warm for comfort in a No.4 action.
You asked, though, not for caliber suggestions but for a barrel. Dang.
