Been pondering a build myself.
I will not build a 223 either as I already have many and want to build something between 223 or 6x45 and 243/270
I need the ability to hunt with it as well, so it has to be at least a 6mm.

I built my buddy a 6mm-06 off his old 30-06 and he loves it. Calls it the "laser"
That won't work for you in a short action.

Have you considered a (6mm or 6.5mm) Creedmore or a 6mm -or- 6.5x47 Lapua (http://www.6mmbr.com/gunweek073.html)
Most of the conversions I have considered require a .308 style bolt face... sorry.

Have you built a 6x45 (223 necked up to 6mm) or it's bigger brothers built on the 222 Rem Mag, the 6x47


Obviously not telling you what to do, just bouncing my ideas that have crossed my noggin