Update:

Tried the Lee .510 bullet sizer last night and was able to get the brass to chamber! Now at least I can fire some cases and send them off to Hornady for a custom die. I’m pretty happy that I’ll actually be able to shoot the rifle!

I purchased three different bullets to try....grease ring and paper patch bullets from Buffalo Arms, and also some 350 gr Woodleighs. Thanks to all who contributed to this post!