Whoa, soldier, slow down.
Once thing at a time, please.
Rifle type sights on shotguns work well for the one-shotgun-does-it-all household. That way if wingshooting, small game hunting or clay games are on the menu, changing barrels gets rif of the sights and transforms the shotgun back to a fowling piece with no visual interference from leftover sight bits. Remington makes a couple of Police 870 barrels that are 18.5" long and have rifle type sights. They turn up used from time to time but not often. Ordering direct from the factory parts department might be your best bet. The 20" slug barrels (smoothbore, not the rifled ones) are a lot more common.
On a dedicated HD gun? Sky's the limit, do what you like. It's a one-trick-pony shotgun, and that's it. BUT... (there's always that 'but,' isn't there?)
The real 'rear sight' on a basic bead sighted shotgun is the stock. The stock (as on any long gun) is going to have to set your face up for a proper cheek weld at whatever height the line of sight (LOS) establishes. The LOS on a shotgun is generally very low over the bore line (BL) and the stock is set up for a proper fit on that low BL. If you want to put iron sights on a gun that are much higher over the BL than the original bead, the stock will still need to provide a proper cheek weld for that higher LOS.
And if you want to bolt on some kind of optic that raises the LOS even higher, then the comb of the stock will have to come up to match if you are to establish a correct and consistent cheek weld.
So it isn't necessarily as simple as just sticking on a new sighting apparatus. There may well be other changes necessary too.
Any good gunsmith can drill and tap the steel receiver of an 870 and put any kind of rear sight or rail on it you want. But first you need to figure out how you want the gun set up so you can factor in the other changes that might well be necessary also, to make the new sighting system fit you properly.
Best advice I know of is KISS- keep it simple, soldier.
Stay Safe,
lpl/nc
