Jim's re-boring business is at
Clearwater Reboring or you can get to it on the Delta Gun Shop site. I've talked to him about a re-bore but have not had anything done by him.
I had my Ruger Hawkeye African re-bored from .375 to 404 (.423") by Dan Pedersen at
Classic Barrel Works. The rifle was very accurate before the work and equally so after. Quick turn around time and very reasonably priced (about $350 a few years ago), including the throating work on the chamber.
From our conversation, he uses the 0.100" per side at the muzzle guideline as well.