You can buy cantilever slug barrels for your 870 from either Remington, Hastings, or Mossberg.
The Hastings are supposed to be the best and the Mossbergs are the cheapest. The Mossberg barrel I used shot great.
Any rings should be fine.
You'll need to buy alot of different brands and loads of the slugs to see which shoots best. It's can be a pretty big price to pay on the front end, but worth it.
My barrel/gun liked Winchester Partition Golds. A buddy of mine shoots lights-out with Winchester Platinums.
Good luck with whatever set-up you go with !!