+1 for the new barrel. It might be safe to use steel in the old one. That is around the time manufacturers were making all barrels tough enough for steel shot. If you get the serial number to mossberg they can tell you if it is safe. Try mossbergs website. I would go with the new barrel just for the ability to change chokes. If the barrel was not made for steel it will eventually wear out the barrel. If your gun is chambered for 3 inch shells no worries. If it is a 2 3/4 chamber
Very bad for both you and the gun. A 3 inch shell will fit, but that does not mean it is safe. The crimp on a 3 inch shell will roll into the barrel and cause very high pressure.