Have you considered starting with a Rem 700 221 fireball? The magazine, spring and follower are already set up for the shorter cartridge. No guarantee you won't need to tweak things to get 100% feeding, but it should be pretty close. Not sure if the short mag, spring and follower is available aftermarket or not.
I do know Remington put the extractor and ejector on the 221 bolts in a different spot than on the 223's, so it's not just the mag that they tweaked.