The .450-400 is basically a necked down .450NE.... If you have enough receiver material to mate with a barrel set of sufficient girth to handle the 450-400, you have enough to handle the .450NE.... The actual barrels could be slightly slimmer and trimmer on the 450-400 once they've been tapered down from the chamber area.... The balance of the finished rifle would be the deciding factor for me........ I'd prefer the .450NE because of the much wider selection of projectiles in the .458 caliber. The .470 and .500NE cartridges have a slightly larger rim and case head than the .450NE.... But only slightly. The .375 Flanged Magnum is a different (smaller) sized beast all together.

It really boils down to two questions, the second of which depends upon the answer to the first:

1. What does a talented and knowledgable gunsmith with sound experience building double rifles deem possible?
2. What caliber do you prefer?

In the final analysis, it is really a question of what makes you happy and how much you'd be willing to spend once the gunsmith has decreed what is "the Art of the possible".