Not having played with the calibers listed I will give one piece of advice. If you are going to build a different one make the change a substantial one.

I only say this because a small change from what you have now may leave you wanting. When Fal kits were cheap many built them in everything from 9mm lugar to 8MM mauser. the cost of the components to load or heaven forbid one has to buy ammo will put a damper on some choices.

Taking a .308 and making a .243 out of it was boring. Taking a 9lb .308 rifle and making a 6lb 7.62X25 carbine was fun.

As far as distance goes with an AR how many shoot past 600? I know a few that do with 80's but in all reality just because it can how many times are people that have an AR going to shoot at 1000 yards vs 300-600?

Just a thought.