7 mag is a strong choice given that you're already working with a LA.
6.5 PRC, is marginal at 600 yards IMO- right at the minimum 1500 ft lbs "consensus" minimum energy for Elk with 143 ELD-X's. Personally, I'd want a bit more energy if 600 yards- or more- was a likely scenario.

The most important factor for you is availability of commercial ammo- and you're as likely to find 7 mag ammo as much as anything else in a pinch.