Looks like you need a 25wssm die for initial neck down. Then after firing resize with a fl bushing die and seat with a 257wby seater. One more step. I’m sure if the cartridge takes off WTO will have formed brass and dies made like they do for some of their other wildcats.
Necking down adg 270wsm brass would be another good option with very little velocity loss. If you know someone with a reamer set up for the heavy’s.
If I was doing another big 25 today it would probably be the 25Mega. Dies and brass ready to go and it’s a simple neck down. WTOs 25-7prc seems just as easy just need the additional 25wssm die for initial neck down.
And the 257wby running peterson brass is ready to go option. Reamers, dies, brass and all.