Buttstocks will usually fit within gauge. You may have to drill out the hole slightly putting an 870 stock on an 1100/11-87, but maybe not. I have seen it both ways. Fore ends, no, as noted.
Oh, almost forgot, the depth of the clearance hole at the rear is different, so you will have to stack washers or make a spacer to get the nut where it tightens up.