GK,
Shame you don't want a 375-284. I have a spare set of Redding dies excess to needs. If it must be a 0.423" ... contact Redding directly as they'll open 416Ruger dies at minimal cost, then grab a 0.423" neck/throater and follow a 416Ruger reamer into your barrel.

The 423Ruger has been built, trialled and is a nice option in a standard length action with minimal modifications.
Cheers...
Con