It would be just a std title one firearm (like a rifle is)as long as either the the bbl or the rec (breach ring and breach ) were demilled. Its only at the point that both are considered servicable that you have a DD.
As far as building it as a 50 or smaller you shouldnt have any problem as long as you weld the insert perm in place before making the firing mechanism live . As long as the bore size hole exist in the chamber there is no problem in removing the blocking bars in the bbl , its still not a bbl just a piece of scrap as long as the large hole is in the chamber.
The only import docs I have seen ,on a 75 mm, didnt require any blocking bars in the bbl , just the hole in the chamber and the threads be removed form the breach ring so the breach couldnt lock in place. I havent seen any specs ,other than recently, that cover the exact methods of demill, I think its always been on a case by case basis , usualy a detailed explanation along with diagrams is returned with the form 6 , and varies wildley depending on when and who issued it.
If had a demilled recoiless I would file a form one . pay the $200 and register as a DD again , would be a good move finacialy if at some point in the future they wont allow such to be registered. Would also make it easier to build with a subcal insert since you wouldnt have to perm weld it in place and you could weld up any nasty holes and at least make it cosmeticly apealing .
What ever route you go keep in mind when welding and repairing that you (or your heirs) may want to sell it someday so dont do anything foolish that would make the demill any worse or reduce its value .
Just my .02
