I bought a police trade in barrel for my WM 870 to shoot slugs. It is of course, Cylinder Bore (i.e. no choke). The Foster type slugs (hollow base) work the best in it, it being smooth bore. With Remington Sluggers I am able to keep them all on a 9" paper plate out to about 50 or 60 yards, offhand. With my home cast Foster slugs in my reloads, I can extend that range to 100 yds in keeping them all on a 12" X 24" target.
I agree with others that getting a dedicated slug/HD SG is a good idea. But if you were to choose to go ahead and cut your single shot SG down as you propose, I think it would work very well for you with Foster Slugs. You can prolly have a bead installed on your short barrel for about 15 bucks.
"Always place your clothes and weapons where you can find them in the dark"
"Understand that the enemy is not the enemy in his own eyes ;this may offer you an opportunity to make him your friend. If not, you can kill him without hate, and quickly."