A buff is a buff, a big tough animal. Some diehards on here will freak out if you don't call it a bison but whatever. I took mine with a Grizzly Stik and STOS head at a total of about 1000 grains out of a 76# silvertip.
However, my friend took one a year layer with an Olympic setup, shooting Carbon Express with Magnus heads. Still heavy draw, I think it was about 80#'s. The arrows weren't nearly as heavy though. He got a complete pass through mid way up the body just about eight inches behind the front leg. It looked beautiful, but two hours later there was a very angry bison still on his feet ready to take out anything that moved. Very dangerous. Quartering away is very important if you can. Stay about a quarter of the way up from the belly right behind that leg and you'll get into the boiler room. I can't tell you that your setup will work as I've nevcr shot one with a lighter setup, but if you put a super sharp two blade broadhead in that spot your chances are increased dramatically. Do not shoot a three blade head no matter what anyone argues. You need as little resistance as possible going into that giant animal with a light bow like that..
As far as the pen thing, the buff is going to get shot no matter what by you or the other guys for meat. You might as well get the experience of taking it with a bow as far as I look at it.
I don't believe the native americans shot 1000 grain arrows out of 80# plus bows, so maybe you'll do fine? But I am assuming here.