If I were doing it, I would have T/C, Match Grade Machine, On Target Technologies or SSK Industries make a barrel for me. That said, you could cut down a rifle barrel for which you would then have to use a rifle forend (which some do and like). You could also have a 338 Federal barrel rechambered to 338 Win. Mag.
Keep in mind that if the time comes you want to sell your barrel to try something else, a barrel made by a reputable barrel maker will keep its value much better than something that has been hacked up and/or rechambered.
Oh, and consider a muzzle brake.