To make the stance work...get the stock shortened, if need be.
Put the majority of your weight, on the balls of your feet, knees slightly bent, lean forward...as if you are almost ready to fall forward. Have your stance ready, before you mount the gun to your shoulder. Have someone make a video of you mounting the gun --- so you can pick-out any mistakes.
Before you mount the gun...have the muzzle pointed towards the target; or intended area of the target. Shoot at least 6 rapid fire shots...if the recoil pushes you back --- your stance is wrong --- or you might need to shorten the stock.
An excellent DVD: Mat Burkett's, Shotgun Mastery