On the front you have the option of several sling plates. The GG&G model, or one of the SGT/Vang/Wilson models for the 870/1187 that position the sling at either side or the bottom.
The other option is one of the mag tube clamps, several of which have a sling mount as part of the clamp. I would recommend either the Nordic or CDM Gear units. Some clamps have rail lengths that you can mount a sling mount to.
For the rear, I really don't like the molded in POS mount. I've seen them hog out with abuse. I use the FSA from Gear Sector.
There are similar items from Blue Force Gear and several others.