If you can' t shoot the 350 it' s not going to do you any good anyway, if you feel comfortable shooting it carry it, if not shoot the 270. you are going on a caribou hunt not the griz. hunt. If your outfitter really thought that you were going to be attacked then he would advise you to bring a bigger gun, he has a business to run and can' t afford for his clients to be eaten by ole GRIZ every time they are there.
Anything can happen but you need to concentrate on the goal at hand HUNTING CARIBOU. Use caution when dealing with your meat and have fun, thats what this is all about isn' t it?