Avid/James,
Just to be totally clear; common "standard" .450 Nitro Express brass has a thin .040" rim. There was at one time "thick rim ".065" .450 3 1/4" basic brass available from HDS, Bertram, and Hornaday. That brass has not been available in the USA for quite awhile.
These days the ONLY thick rim brass (besides custom turned from Rocky Mountain Cartridge) would be the .450/.400 3" Jeffery.

Easy enough once you actually find some of the Jeffery brass. Trim to rough length, anneal, fireform with ~10 grains of Bullseye and topped with Cream of Wheat and capped with paraffin plug or whatever. Trim to final length, maybe inside neck ream. There you go.

Likely the fire formed cases will be ready to load and fire with the chosen load.

Depending on the rifle & load you may be able to neck size only using a die that happens to work for the desired neck diameter while you wait for custom dies from ch4D. Chances are that the recoil is not enough to have to worry about crimping.

Good luck & keep us informed of your progress.

- Mike