Well I agree with the no head shots, I'v always been told to avoid them because the margin for error is to small. Why aim for such a small target when the chest cavity offers a much larger kill zone, heart an lungs. As for needing a larger Caliber, well I guess it depends on where you are from. Here in the yukon the minimum for bison is center fire .30 caliber or larger with minimum 180 grain bullet; the 30-06 is the base line.
My old man has been hunting for the past 40 years and has only ever hunted with a 30-06. He has successfully hunted everything from sheep to bison with his 30-06, he had a 375 h&h which he never found any use for because his 30-06 fulfilled all his hunting need with out failure so he gave it to me

. Nothing wrong with going with a larger caliber but you may not need it and the larger the caliber the more expensive the ammunition. Check and see what the minimum is in your area then make you choice based on that. For up here you would not be allowed to use a 270 for bison even tho it shoots just as well as a 30-06.