Daryl, I linked your wildcat to a guy on AR who was looking for a 2.5" 284 case to make a 338 on. Using your design and basic brass and it could be made darn close to a 2.5" 284.

While I have never heard of a 458Wmag having issue with headspace I admit to not owning one.

Michael Petrov address all the headspace rumors for the 400 Whelen in his 2001 Articles.

I love it and have never had a problem. But everyone needs to shoot what they are comfortable with.