A couple things....
I too have heard that 870 extensions will work with an adapter/spacer. But, if you're going to add a LONG extension, why lop the barrel off at all?
And, since this is recoil operated, reducing the barrel weight by shortening it will mean increased battering of the receiver, especially with heavy loads.
Also, since this is recoil operated, the mag tube cannot be clamped to the barrel, so it's much more vulnerable to damage.
That fiber buffer in the receiver is at least 60 years old. New ones are around, and it makes sense to add a new buffer and replace all the springs and friction parts.
Were I putting together a practical model 11, and I've toyed with the idea for years, it would have that ring/spring/buffer makeover, a two shot extension for 6+1 capacity, a good pad and at least 23" of barrel. That barrel would have a long forcing cone, a night sight and Mike Orlen, Carlson or the Brileys would install choke tubes.
Oh,yes, welcome aboard......