I think I run 320 mains on my 600 VES out there, and I'll go with 310's if it's warm. You should be plenty rich with the 320's, but you'll run stronger with the 310's. (Just don't hold it pinned too long trail running back to home base if it's gotten cold at night.) Definitely put your needle clip in the second groove from the top if you haven't already.
I've pulled the stock weights (58's) out there, but I'm geared down (I actually don't know if gearing makes a difference with how you clutch, but I'd think it would.) Chart says you should run 54 gram weights at that elevation, but in my experience, you definitely shouldn't need to go below a 56 gram weight.
I really don't know anything about clutching for the Shift. Polaris usually only has you change weights for an elevation change.