There's zero practical difference between the powder capacity of .270 and .280 cases, in fact it's so small a difference in the weight of brass can cause some overlap. Also, one of his requirements is fairly easy to get brass.
My vote would be for the 6.5/.270, whether standard or AI. There's no ballistic difference between it and the 6.5-06 or 6.5/.280, but the neck's the longest of any, reducing throat erosion. And you can just run .270 brass into the sizer and not have to trim off a bunch, as you do with the 6.5/06.
