A good gunsmith can make you a 400 Whelen. Trick is to find a reamer. There is the original 400 Whelen, the newer Petrov version (basically same as the original), the
400 Brown-Whelen, the 411 Hawk and perhaps some others.
Headspace problems are a myth, as long as the gunsmith knows his stuff. Ballistically it is about the same a a 450/400 NE 3", a round that has long been popular in Africa,
for everything.
So there is plenty of power using 300-400gr. bullets.
Making the rounds is easy, best is to use 30-06 cylindrical brass, but 30-06 or 9.3x62 brass also works well.
My 400 Whelen (actually the 400 Brown-Whelen) was built a few years ago by Dave Jennings in Pritchard, BC, on an old, shot out Mauser Oberndorf.
Excellent work and it shoots exceptionally well (with peep sights). Dave is now retired, but there are many other good gunsmiths.
There are lots of components available and plenty of info on the web and elsewhere.