Longer magazine boxes are made for larger cartridges, so probably won't hold a .223 once the bolt is retracted. Then there's the problem of the .223 follower flopping around in the longer box. You're faced with trying to adapt a .308 family box and follower to a much smaller cartridge, so its not as simple as loading a long OAL .308 in a .30/06 magazine. There might be someone who commercially makes overlength .223 boxes for the Tikka, but if so, I haven't heard of them. The simplest solution is to single load the long rounds, but you could get a gunsmith to weld an extension to the existing magazine box. The bolt stop might require modification if the magazine box was extended, but that's a discussion you'll have with your gunsmith. An extended magazine box could be your best option; the extended box would still be shorter than the next longer factory box, and the follower should still work.