Well, there can't be ALL that much involved, because if you send your 870 to Wilson Combat for their "Remington Steal" upgrade, they will "Replace all worn and/or damaged parts, Convert to 3", Convert shell feed to flexitab anti-jam system, Install new spring and follower, Install new synthetic butt stock and foregrip, Replace safety with current design, Strip and parkerize all external parts, and Strip and anodize trigger group." And all for just $189.
This is not an ad for Wilson, just wanted to point out that since Wilson doesn't do ANYTHING for NOTHING, you seem to be getting an awful lot--including converting to 3"--for not a whole lot of money. This leads me to believe the conversion to 3" piece of it isn't all that bad.
Scott