NO ! you do not need to turn or ream the necks by going from an 06 basic to a 400 Whelen. That is pure BS. Perhaps if making 25-05s (why would any one ?)

375 and 400 Whelen require NO modification of the case other than checking the OAL.

I have also made 375 and 400 H&H from Norma belted basics, no reaming or turning needed !

The key is correct fireforming as I outlined above.

Don't buy any dies for 400 Whelen until you are 100% sure of what you have. Once fireformed to the proper chamber (G&H) I have NEVER had to FL size a 400 Whelen. When shooting cast bullets I usually use the 41 S&W dies in my progressive press as they size the neck just fine, have the neck expander to ease seating and a crimp die if shooting heavy loaded hard cast bullets.

As Packard used to say: "Ask the man who owns one."

These were made from Norma Basic w/a 350 X bullet.