There isn't too much dificulty in modifying a shotgun. In fact you can cut the barrel down yourself if you take some care. Personally I would stick to around 20" +/- for the best ballistics and sight radius,,, owners choice I guess.
I have cut down a couple of guns in my life and what I did was to use a plumbers tubing cutter to lightly score a line around the barrel to follow with a hacksaw (tape both sides of the cutters wheels to protect the barrel). I carefully cut the end off and filed it as square as possible. I found a large metal rivit at a steel supply house and put some lapping compound on it and using a drill, polished the inside edge of the bore to create a smooth crown of sorts. The outside edge was carefully filed and sanded smoth and a little bit of instant blue, Done.
I would consider installing new fixed sights, front and rear and a good recoil pad. If I remember right the Mossberg and others have a split rear bridge so scope mounting wouldn't work. I hope this is of some help with your project.