Here are two images, the first of a modified Surefire forend, and the second of an unmodified Speedfeed forend, both installed on 870 shotguns fitted with our 8-Shell Holders:
As you can see, the shell holder goes right up to the edge of the receiver, so any forend that extends over the receiver, even a tiny bit, will have to be modified. The Surefires extend over the receiver about 1/4". We cut about 1/2" off the end of our Surefire forend. The Speedfeed requires no modification.
The factory Remington forend (NOT the police tactical forend) is so long it needs to be cut even to fit with our 6-Shell Holders.
We recommend simply putting the forends in a miter or radial arm saw fitted with a wood blade and chopping away. It only takes a second, and a sharp wood blade leaves a clean, almost factory appearance.
When we modified our Surefire, we mistakenly used an old pipe-cutting blade that someone had put on our saw, so it wasn't a very clean cut. But a few minutes with some sandpaper restored its tidy appearance.