I converted my 870 WM into a double duty SG. Went from a +3 to a +2 to a +1. The longer the extension, the more critical a mag tube clamp. Some have bumped it and caused quite a bit of damage and needed to replace the guns tube.
TacStar sidesaddles are good, they come in 4 round and 6 round. The 6 round is what I used and I had to trim part of the forearm down to use the wood. I didn't like the corncob replacements they sell. When you mount the SS, do not overtighten it or it will bind the action.
New barrels are too much $$ (for me). Look around for a police trade in barrel. I got mine for 50 bucks. Scout the boards as they get posted about on them.
I got the bigger button safety also. Makes releasing the safety possible with the side of your trigger finger without changing grip. I like it.
I also added a Blackhawk ammo pouch / cheekpad to it and cannot recommend it more highly. I bought it for the ammo carrier but like it more for the cheekpad. Really saves the cheek in heavy recoiling loads.
Remove the extension slowly and it wont launch the spring!
HTH's