Well you now own one of the most prized versions on the 870, highly prized by upland bird hunters, some would pay a kings ransom to own one. The straight may make slug shooting dicey for some, slug barrels can be found on e-bay most weeks, be prepared to pay. You'll need to decide on a rifled barrel for sabots only, sights or cantilever for scope mounts. The second choice is a smooth bore slug barrel with sights. In your case also an option is to use your current barrel with Foster type slugs or buy the rifled choke tube and use sabots.
On my Wingmasters I shoot a smooth bore barrel with Winchester slugs, I have a Williams set of sights that mount on the rib for deer season. You'll need to check to see if the standard 20GA barrel will work on a 20GA LW frame. Your best info will be the Remington forum at Shotgunworld.com, post a question, "Virginian" over there is very knowing about all things Remington, also a Slug gun forum, I post there under the same screen name as here. Happy shootin'.