What long action, with rails for the Ultra Mag will not reliably feed the WSM's?
Cycling slow and fast, I've checked the Rem 700 300 Ultra action I have and it feeds the 300 WSM
case flawlessly, 1st through 3rd round. The first and second round fed both enter the front receiver ring up to the base of the shoulder before they can leave the rail. By then they cannot pop out, as they are too far into the chamber with the bolt backing them. They are actually quite docile entering the chamber using the standard Ultra Mag follower and spring combination, and the spring is stiffer than it needs to be. Pressure is balanced more tot he front of the WSM case, as it's designed for the Ultra Mag, but it works flawlessly in the 700. Upon firing it, you may find the cases drop low in the rear of the mag, but I seriously doubt they will. I would make sure to chamfer the hole in the bolt so it doesn't drag on the shoulder of the WSM case and force it down at the rear when withdrawing the bolt, but again, I doubt it would, a belted design would be worse. Cases falling low at the rear would be the thing I'd watch for, but it could be fixed easily if it happened.
The last round fed is held under the rail 3/8" beyond the rail transition.
See how the Savage feeds with the shorter WSM's, then compare it to the 700 action and then decide. I'll have a look at the Savage when I'm out at the gunshop.
The new Savage actions have 1-1/8 thread diameter over the 700's 1-1/16 threads, so you could open the threads up in the 700 for the Savage barrels and use their barrel nuts on them, if you wanted to. I'd pin the lug on it, and have them all headspaced for your action as you normally would, use a T-handle action wrench and a barrel vice and just hit 20-30 lbs torque myself, Savage or Remington if you plan on using custom barrels.
I believe the Ultra Mag and Short Mag chamberings in the Savage now come with the bottom of the bolt nose ring removed for semi-positive feed, so they feed the larger diameter cases easier, what I read on Sharp Shooter's Supply website I think.