I just hauled off and D&Ted my 870 20 gauge myself without seeking out Remington's advice. If the actual number of people that have done this could be polled, I would be surprised if ANYONE has had any trouble at all with the screws or bases coming loose due to issues with "thin metal." The trick, I believe, is a combination of using the right screw size and thread, the correct application of blue loctite to the screw threads and proper screw torquing.
That's my opinion, for what it's worth.
FullChoke