Do yourself a favor and protect your right to ownership. NO SAWED-OFFS.......... Found a 535 used on Ebay, 22 inch rifled slug barrel which goes back to my question about sawing it off myself. I have a hack saw .....
If you want to purchase an SBR or Other, or have a reliable gunshop do a legal conversion.... pay the price and do the forms.
BUT you may be able to convert a shot barrel to a slug gun barrel by having proper sights added to a shortened barrel. Ask a gunsmith if this is possible IF you can find no other barrel option.
I suspect once you add up $$$s, it will be more cost effective to buy something more suitable.