I originally debated on making an M40A1 in .223 as well but I've decided to hold off on that. Besides the cost of the build itself, the cost of the ammo isn't that much different I've realized now that I picked up my Howa.
On average, the .308 Match I shoot now, run about $40 a box. The .223 Match ammo I just purchased is only about about $5 cheaper per box. Not a whole lot of huge savings and really target shooting with a 223 at the same distances as the 308 isn't as easy, especially if there is any wind