18" barrels. 26" overall minimum under Federal law and here in Texas. Make sure you do not have something more restrictive in your jurisdiction.
16" barrel. 26" overall for rifles.
Both are measured from the breech face. It is always safer to go 18 1/2" to be safe. A gunsmith can cut and recrown the double barrel, install a new front sight bead and dress up the finish.
Keep in mind, you will not have a choke at that point. Your barrels will be Cylinder Bore. I believe Doc Holliday had a 10 ga shotty.