I've also got a 870. I ordered a weaver base from RR arms in minnesota. It's made for the 870 super mag which comes drilled & tapped, but it also fits the 870 mag express. The part # for the base is 429M 48454 matte. you will also need a # 33 drill bit & 6-48 taper tap(395-648-101). These you can get from brownells. I just used my drill press and some straight edges and a sharpie marker for layout lines. I finished off the installation with weaver medium grand slam rings and a cabela's 2.5X pine ridge shotgun scope. i killed the only turkey that I saw this year at 55 yards with #4 hevi-shot through a rem hevi-shot choke.
A red dot sight or a compact scope like the truglo 4x won't fit because the cross slots in the base are'nt set up for it. if you choose one of these options I would suggest a picctinny rail base, badger ordinance and others make these, they are pretty expensive about $40-$60. The weaver base is about $5.