I would be real nervous about shooting a mechanical broadhead with a 56 pound bow. Do you know what kind of Kinetic Engergy you have with that setup? If it's less than 60 lbs KE I would stick with a fixed blade to be safe.

If you are going to shoot a mechanical with that bow I'd pick a 1 1/4" head. Stay away from the 2" cut because you won't get good penetration.