Bill, I do have some answers here. this is a copy of the post that I put on another forum that I frequent.
"An update from the kitchen table gunschmidt.
First note, the 870 furniture that I found was Rem. factory composite (plastic) and I had picked one set up on Friday. Yesterday, I pulled down the 1100 and compared the take off butt to the original.
Both have the same profile and a through hole that will accept the bolt/action spring from the 1100. The only real difference that I could see was that the tennon that mates with the reciever is about 1/16" to 1/8" shorter on the 1100. After some jucicious use of a 4-in-hand file and belt sander, the tennons matched and the but stock went onto the 1100. I still need to relieve the bottom of the compression tube (supports the stock bolt/action spring) to close up a 1/32" gap at the top of the reciever where it meets with the butt stock. In fact the new stock snugs up better than the old, damaged wood one that came with the gun.
Now we have some answers for anyone else who might be thinking about this."
By the way, I checked the depth of the material in the tennon/wrist of the stock and it was solid plastic deeper than my 1/23" drill bit is long

Will