Given the choices, I'd go 300 win mag. Probably short mag. For best accuracy with reloads you want to head space off the shoulder anyway (my and my hunting bud's .338WMs both showed a significant improvement), so the belt becomes superfluous. The short mag can almost equal the mv of the WM, but that too is negotiable and largely unimportant within limits. You want to go for best accuracy, whatever. MV is way over-hyped- again, to a point. Danged near anything will kill to 600 yards, if properly placed.
Know the range (range finder) and the drop of your load, and you are home free.
Factor in wind of course.
Personally, I'd go with a .30-06 with a moderately heavy barrel. It will do anything the 300, 7 Mag, or .338 will, with what should be obvous advantages, and few if any signifcant disadvantages. My heavy barreled '98 in '06 is zeroed at 300, producing from solid rest, 3 shot groups right at 1" plus or minus at that range. I'm pretty sure I'm the inconsistent factor- and limit myself to 500 yards on game. So far.
The rifle is more capable than I am....I haven't shot the .338 in 8 years.... :and it is a 1.25MOA or less, 500 yard capable rifle. but then, I've not been in .338 circumstances, either.
Most people shoot lighter calibers (and heavier guns) better (other things being equal), even if they can handle a boomer's recoil... fact of life. You will have to decide where your compromises lie.
