I just invested a year of time and a good chunk of $$$ building me an elk rifle...its a 30-06...because, while I can shoot a 300 mag just fine, I simply don't enjoy shooting it as much.
You don't need a magnum for 350 yard elk...a properly loaded 30-06 will work just fine out to 500 (even 600 in the right hands).
Pay attention to the weight of the rifle...lighter is better (easier to carry at 5,000 feet), at least up to the point that it gets painful to shoot.
My rifle came out at 9 lbs. even...and in a 30-06, its a dream to shoot, no problem at all...it may be a bit heavier than many of the so called "mountain rifles", but not so much that I can't carry it...light enough to carry, heavy enough to shoot...thats where its at.
If you really want a magnum, get one...but you don't "need" one.