What really was holding .260 rem back originally was good brass, but you could find good brass for 6.5 creedmoor. For years now you have been able to find good brass for the .260, so that is no longer a drawback. What makes .260 better than 6.5 creedmoor is the ability to push the case more with hotter loads, squeaking out a little more performance than 6.5 creedmoor.
Then you also have the fact that you can use .243, .308, and 7mm-08 brass for the .260 rem if you are a reloader. .260 rem is better for experienced reloaders because of this. 6.5 creedmoor is better for beginner reloaders because Hornady shares their exact load data and also better for those that really don't want to reload because finding good 6.5 creedmoor bullets from the store is easier.
I'm surprised nobody said anything about .243 Win as being a flat shooting cartridge that could be used for Ken's build. Now it only has enough energy to humanly take deer at about 400 yards in my opinion, but some people have taken deer at greater ranges with it. And if you are just shooting steel at long distances, a lot of long range shooters love .243 because it is a high velocity flat shooting cartridge.
http://www.6mmbr.com/243win.html
Ken, now I know you talked about a big flat shooting cartridge for your build. As you can see from the posts, flat shooting and big don't go hand in hand if you are building off a .308 platform. If you are fine putting some more money in a bigger AR platform, there are bigger flat shooting cartridges out there such as the 300 norma that someone previously mentioned.
Good luck with your build!