I recently put a speed feed 4 on my 870P. It is the short stock version. I think they make some of the highest quality stocks for this. I believe I ordered from RR Sports on the web (do a search for them and pick the cheapest). About $85 for the set. I love this set up, but kinda miss the standard wood it came with.
Mine doesn't have the shell holder in the stock, but I'm curious to see how well that works.
Best, Coop