if there's a good industrial hardware store in your area you can buy a pair of Jacobs Chuck removal wedges. They pretty cheap and have a shallow taper so they don't require much force. When used in pairs between the back of the arm and bottom bracket, they'll gently and evenly push the crank off with zero damage to either.

In fact they'll work so well that you'll feel confident enough to continue using the stripped crank with these as the remover of choice.