Back in the day when Mdl-12's were the thing, it was a common known gunsmithing operation to open/lengthen the ejection port to accommodate 2 3/4" shells. This is easily done on a mill & depending on how picky you are the freshly cut metal at the front of the port can be cold blued for a close enough match without the need for a re-blue of the receiver.

As mentioned above the easiest & cheapest route is to just use shorter cased shells in it & have fun with it.