Here is a link to all the info you need on shot placement on a bear. You should shoot farther back than on a deer as the bear has a larger scapula. I used a Wasp Boss 100 4 blade to take a Bear about 150 #. The broadhead worked great.
Good luck bear hunt!
Here is the link: http://www.theidahosportsman.com/bear%20anatomy.pdf
Scapula is the front shoulder blade. It is very dense bone and hard to penitrate with a bow. I've seen shotgun slugs stopped by scapulas on large deer of coarse that was on a loger shot. It is located at the top of the leg and can protect the heart and lungs if the leg is back. Heres a picture. http://home.mn.rr.com/deerfever/Anatomy.html
OK, I see!! It seams the best shot is front leg forward and wuartering away slightly. That is easier said than done when a 500+ pounder is only 10-30 yards away fro ya Thanks-Jack.
I have taken 3 blacks with my Vixen with a basic whitetail setup. The bear weights though were 250-400#. Also know many archers that use their same whitetail setup for black bear also. You should be golden with what you have. Your best shot like you said is in the armpit of the front leg when it is extended. Although any double lung shot will smoke'em also. If its 800#'s you just have a bigger target. GOOD LUCK.