Updating my reply for more clarity...
There are two styles of 870 stocks - early and late:
To fit the early 870 onto an 1100, the bolt hole has to be increased in diameter. Then you have to put a short sleeve under a washer that would be under the action spring tube nut to space it further out.
The late 870 stock fits an 1100/1187 with no modification needed.
To put an 1100/1187 stock onto an 870, you need a special longer 870 stock bolt. I don't recall the part number, but the pic above shows the length difference.