IMHO, a shotgun is best fitted with a rib and dual beads. Unless it's to be used as a rifle with slugs or buck shot. In which case iron or optical rifle sights are appropriate. In HD situations the shotgun is quickly pointed, not aimed, and elaborate sights are superfluous.
That being said, let's see what what we can do…
It seems you wish to duplicate the sights on the current M16A4 rile and the M4/M4A1 carbine. These units utilize the old adjustable square post front sight introduced on the M16A2 and the flat-top receiver has a Picatinny rail for the attachment of optical sights or other aiming devices. So, your mission is three fold: mount an A2 type front sight, mount a rail on top of the receiver and select an appropriate device as the rear rail rider. Also, for the M4/M4A1 look, a telescoping stock is appropriate.
A correct looking A2 front sight is the key to your project. I'm not aware of a direct fit for the 870; but,
Brownells has rail mounted A2 type front and rear sights. So, next you must decide what kind of rail system you want: a receiver rail and a front rail, or one continuous rail -- the choice is yours. Once you get the rail/s installed and the A2 clamped on up front, it's just a matter of providing a rear sight to suit your needs that's appropriate to the M16A4/M4/M4A1 modular system. It's a simple choice:
[SIZE=-2]graphic courtesy of mfingar[/SIZE]