I don't know if this'll help you with your 870 mods, but here it is:
Mag extension - especially if you want to keep the longer barrel:
http://www.midwayusa.com/viewProduct/?p ... ber=131077Barrel:
http://www.midwayusa.com/viewProduct/?p ... ber=921370Just a few options for stocks, etc:
http://www.sportsmansguide.com/net/sear ... ington+870I once had a Remington 870 Magnum, had a TacStar 6-shell side saddle, the 5-shot extension, 28" vented rib barrel, and Knox Stock setup on it. It was worthless for home defense in the sense that it was SO long that "quick handling" never entered into the description - and I love my shotguns. Heavy as all get out when fully-loaded and completely pointless since you could better outfit a much shorter and lighter version and accomplish the same dang thing with half the weight. I can also share with you that I liked the TacStar side saddle - it took a beating and never cracked, held onto the shells when running, tripping and falling,

etc, yet still let go of them without needing hurculean effort. The Knox Stock was ok but in my opinion I'd go with the lightest butt stock you can find that fits you and invest in one of the new Supercell recoil pads from Remington - also check out their somewhat-new screw-in chokes for stand-off/tactical applications. Kinda cool. Also make sure you check the staking on the bars inside your receiver - if stake marks look a little puny give Charlie's a call and see what the gunsmith would charge to restake them - it's worth the $15-$30 to have it done so you don't risk it falling apart when you need it most.
Anyway, there's my input for what it's worth.
-Crunch