I don't know for sure, but I think the action length is the same. I have two short actions and a .338 WM, so I don't have a standard long action to compare with.
I think the only mods are opening the bolt face and add a muzzle brake on the new barrel...
I'd be happy to measure the mag box, ejection port, or anything else that I can on my .338 for you for comparison's sake.
You could try PM'ing Coyboy too. He seems to know them pretty well.