Just order or find a complete upper in the caliber you want. If it's the .243 wssm. you want, then so be it. BUT surely you understand that a .243 is not the same as .223/5.56. Two completely different calibers, meaning two completely different sized bullet dia. and two completely different sized bolts needed for the different sized brass.

Just a side note, if you're not into reloading, the .243 wssm. may not be your best choice, and if you're new to reloading, the .243 wssm. can be a little cantakerous to reload for.