I've put together several 870 expresses in 20 and 12 gauge using 21 inch vent-rib barrels I bought from Midwest Gun Works. I like the way they handle, you can get them through a house as good as an 18 incher, IMO.
This is a 21 inch with slug sights:
https://www.midwestgunworks.com/page/mgwi/prod/F243190
I'm not sure where you would be on price: $88.00 to $120.00 for the Express sights -
https://www.xssights.com/Products.aspx?CAT=8271 plus dovetailing the rib for the rear and installing the front sight; plus the expense of cutting down and finishing the barrel.