I have also done a similar conversion with a 12 Ga. SxS. I would suggest you find a load that shoots good groups from each barrel, even if seperate, before shortening the barrels. In my case my gun shot groups about 8 to 10 inches apart at 50 yds. I then started taking one inch sections off the barrels until the groups converged. By shortening the barrels you are reducing the time the slug stays in the barrel as it recoils up and away from the center line. I put a rear sight on the rib and superglued a front on, then as I chopped of the barrels I'd knock off the front sight and reglue it on the muzzle end of the rib and shoot another group. Started with 28 inch barrels and ended up with 21 but it would hit a playing card at 50 yds with either barrel. BTW did the shortening on the range with a hacksaw, drew some interesting comments!!
That Lyman slug mentioned is a real killer and accurate. I've probably taken 6 - 8 deer with them and never recover one, complete penetration. I cast mine from wheel weights.
Best.