With only 50lbs of draw weight I would not suggest you shoot a mechancial head that large. In fact the only two that come to mind for your setup woud be the Rocky Snyper or the Rocket steelhead. Both of these are smaller mechanicals that penetrate well. To be honest you would be better off overall with a smaller fixed blade. Something like a Magnus stinger, montec, Snuffer ss etc. check out my website and read through the free pages, it has a lotof info on picking the right broadhead.
www.broadheadtests.com