Since it is now a "shotgun", meaning barrels longer than 18", overall more than 26", and having a buttstock, it can only be reclassed as an NFA "Short-Barreled-Shotgun", or SBS. Of course pending legal in your state, and by following federal regs such as paperwork approval, $200, and engraving.
Old doubles have some collector value, you might be able to trade it off for a newer 12g and come out slightly ahead and have much more choice and availability of ammo.
Please post pics when you are done. I've wanted to do one too, but have too many other projects.