You guys speaking of using Pac-Nor barrels should be aware that their shop burned down a few months ago. I believe they are still rebuilding. As far as I know they are not open for selling barrels. They have the announcement on their website
http://pac-nor.com
Other than chamber you need the same spec as a 375H&H. 1-12 twist.
Have you considered a profiled barrel that a smith can chamber. You could use a shouldered barrel and lose the barrel nut so it would not need to be turned down.. Frankly on the .550 dia magnum I would welcome the extra steel around the chamber.
I am waiting on a barrel order that's planned for a 375 Sherman Max pack gun 22" barrel. Nice and handy and plenty power out to 600yd. 260 AB or 248 HH @ 500yd they hold 2k fps / 2k ftlb which is good for anything in NA For longer range the T3 action and Sherman (RSAUM based)case allows even the super long 300gr CEB MTH @1.8" to fit and should retain 1500fps@1k. Lighter weight 235 Speers HC for deer.
Unless your sold on the 375 and using wsm cases the 338SS has a good amount of perf data and much better bc bullet choices for longer range and easier to find a savage style barrel profile. Fits in a true short action mag.