I have looked into this myself a few years ago. All my slug barrels are 3" chambers and I use them on some 2 3/4" receivers. Remington says in the instructions that come with the barrels that they will fit either but to only use 3" shells in a magnum receiver.
If you compare the parts list for a 2 3/4" and 3" gun, the reciever is the same part number and the only difference is the ejector as Remington said. The ejection hole in the receiver is the exact same size.
There are companies that have advertised making this conversion for years. Ballastic Product was who advertised it and I talked to years ago.