Does your 870 have a "tactical" fore arm. You can see an example of the "tactical" fore arm at http://www.remington.com/products/fi...-tactical.aspx
My 870 Express came with one so I got a "sport" style fore arm like the one depicted here http://www.remington.com/en/products...ic-18inch.aspx
The sport style comes back approximately 2 inches more than the tactical.
While I prefer the LE fore arm for home defense, when a smaller person wants to use it I put the Sport grip on so that they don't have to reach as far with their off hand. You might want to look into the forend on the cantilever model. Of course, I imagine that you have a wood stock that you would want to match.