What length is the barrel on the upper? Best bet is to buy a pistol configuration lower or a stripped lower. If it’s built as a pistol from the get go you’re good. You can go pistol to rifle but never rifle to pistol. That said, you can put any length upper receiver on a pistol lower. SBA3 and 4 “braces” would be your best bet for comfort. If the OAL (from end of barrel threads to end of recover extension) is greater than 26” you can put a VFG. Minimum barrel length required to achieve that is 11.5”. Any shorter and you can only use AFGs.
If I remember correctly, if it has a folding adapter you measure from barrel threads to adapter for OAL.

You can have a MD pinned and welded to make it meet the 16” barrel requirements but you need the right length MD and barrel. Shortest barrel that comes to 16” would be 13.7”
I hope this helps and makes sense