Quoted:
I have a 500 with a wood stock. I cut about 3/4" off of it and replaced the buttpad with a Pachmyr for a Rem 870. Fit perfectly.
Take you stock to a gun store and see if you can find one that will fit before you cut the stock then cut it to fit the new pad.
In the Art and Science of Shotgunning by Bob Brister, he recommends adjusting every shotgun to your specific sizing. He advocates grinding the rubber butt pad. He even suggests that rubbing the buttpad on a concrete sidewalk will do in a pinch. For most folks, great sizing changes are not needed, and furthermore the heel or the toe of the pad may need the attention.
The Mossberg pad looks to be at least 1 1/2 inches. I think mine needs a little removed from the toe "bottom" of the pad. Maybe 1/4 to 1/8".
Any competitive shotgunners on here?