Weaver makes a 2 piece base that fits factory drilled and tapped 870, 1187, 1100.  Stock # 48346.  I just picked one up at Wallyworld for $6.00.  I see alot of people buying the full weaver rail and then cutting it down.  This is a simple solution.  I'm gonna be mounting my FF2 using the pictanny/weaver to my 870.