That looks nice. The 308 would be nicer in a gas gun for recoil as you say. If I was to get/build another AR, it would be a 6 or 6.5 variant of some sort I think. I do have a 9mm in an AR platform that I really like.
What are the measurements of the shields? If we can figure out that they are the same as several other cultivators, you may have better luck finding some.
I have a # 463 rear mount that look very similar is why I asked.