Get a new, threaded barrel for clays, and keep the old barrel to maintain the original configuration. You can get a replacement barrel at Wal-Mart or BassPro.
I just bought a Nova Max-4 12 ga. w/28" barrel. I have 2 questions- does anyone sell a cantilever rifled slug barrel for it (any color)? Second, can the stock be shortened? I need a 12 1/2" LOP.