As others have stated; Have that guns barrels measured for thickness.
Have a read of a thread here, now on page 3, about shooting 2 3/4 shells in 2 1/2 chambers. I consider the little lengthening of the chamber to be as safe as what is being recommended in that thread. If not over reamed in the chamber or overly cut in the forcing cones or had a heap big honing of pits done to it then only little metal has been removed. Measure chambers, measure forcing cones, measure bore, measure barrel wall thickness.
I personally own & shoot a 12g British hammer gun with 1 1/8 Oz proof & chambers lengthened from 2 1/2 to 2 3/4. Now out of proof.
In respect for the guns original proof shot weight & the lengthened chambers I load & shoot 1 Oz low pressure loads for it & have no problems. One ounce is actually a nice shooting load & frugal on powder & shot whilst still getting the job done in this light game gun of mine.
If I want to shoot high, long, heavy, hard, & loud I have other guns with that capability & use them. One friend named one of them, "The Master Blaster" & another friend named another one "The Howitzer"
O.M