The least expensive (best) way to go would be to have it rebored to 338 Federal (338-08). I had a 700 300 H&H redone to 400 H&H and even the cartridge marking was redone so that it looks original. It was still very accurate (1.5 MOA w/400 Woodleighs and a 1-4X scope). You don't have to reblue, fit the bbl to the stock, etc. etc.

My 400 H&H: