Sorry for the delay; I just don't remember to check my messages on this board for some reason. I do size in a .204 dis as an in intermediate step but have not had to turn the necks. I would think going all the way down to 14 cal neck turning (and maybe neck reaming) would be required. I was lucky with this rifle as necking the 218 brass down to .172 was pretty painless.
Sounds like a fun project. Good luck with it.