No Remington Autoloader will accept a barrel from an 870...
Well ... let's say no Remington 1100 will accept a barrel from an 870, just to be squeaky accurate. An 870 barrel will work on a Remington 878, if the proper gas ports are drilled in the 870 barrel.
I doubt the stock will transfer either, but I'm not sure. The Blackhawk site should tell you though.
Frankly I don't see an 1100 as an HD upgrade, but I like 870s a lot, so...
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.