Well I have not shot one of the brown ones with big teeth, but i have shot many big black ones, and yes out of the guns mentioned i would take a 50 yard broad side with the 45-70 and 350 grain barnes or a bonded bullet.
However I have built a gun for brown bears and infact it turns 300 lbs black bears inside out, that i would love to try on the browns sme day.
Its a Win pre 64 , 26 inch custom barrel in a 350 mashburn that chronys a 225 grain Nosler part at 3250 fps. Not the most pleasent gun to shoot all day, but if you hit where your supposed to , the cartridge will do its part.