Decide on a budget first. And don't expect to see the finished rifle for at least a year. Otherwise, a Whitworth is just a commercial Mauser. It's usually best to use whatever brand the smithy sells. The guy might have a .375" barrel blank in stock. The brand doesn't matter much for a hunting rifle. Other than cost differences.
You want a guy who already has or knows he can rent the Ruger reamers too. If he doesn't, you'll be paying for 'em.