So here's what I did with all this good information.
I got a Choate extension tube that gives me a total of 8 rounds. It extends the mag tube to the end of the barrel, but no farther and that pleases me from an aesthetic perspective.
I also installed a synthetic forend with Picatinny rails to match the synthetic stock already on the shotgun.
I don't know the complete history of this gun, but the original stock had been replaced with a synthetic one at some point. However, the original wood furniture was retained at the forend. It looked cobbled together and was unpleasing to the eye.
And the Picatinny rail gives me a place to mount a tactical light, and I like that idea on a home defense weapon.
The result is an 870 that looks good and has a bit more mojo than it did a few hours ago. I haven't shot it yet, but I did pump several mag loads through and found no difficulty feeding.
So, my "gift" shotgun has now cost me a little over $80 and has become another "keeper" gun.
Thanks for all the good advice. I'll se what I can do to post a pic in the near future.
Crusader Rabbit