All that's needed is to weld enough tubing onto the end/nut, with over .410" I.D. (in the OP's case) and long enough to bring the bbl to an OAL of 18" to meet the US Federal minimum bbl length requirement.
The extension needn't be an active part of the bbl, just a permanent extension of the guns length: 18" bbl with an OAL of 26".
If the buttstock's too short to make the 26" OAL minumim, make the bbl extension longer until the minimum's met.
(Why a shotgun with almost no buttstock can be legally NOT a sawed-off - as long as it's bbl is long enough that the assemby meets the 26" minimum, you're golden)