Re: suggestions on caliber
Was thinking the same thing.
Only way your going to achive that is by going with some insane overbore round such as a 25-300 RUM shooting close to 4000fps w/100g pills, and being 3" high at 100.
Your 264 stoked with a 120g nbt would be about as good as it gets for flat shooting.
25-06 with 100g nbt at 3400fps or so is also a pretty dang flat shooter.