IF, and only IF, the gun is properly checked and passed, then going to RST or Polywad for "Vintager" loads in your proper chambering - most likely 2-1/2"- would be the safe way to go. Those loads are VERY low pressure designed for these older guns that have been deemed safe by a qualified gunSMITH - not some parts changer.
A gun that old, I would bet, also has the shorter chamber, something that qualified gunSMITH can tell you.
Remember, the length of a shell is the fired length - an unfired 2-3/4 shell will fit into the 2.5 inch gun, but upon firing, the petals will open in the forcing cone raising pressures to dangerous levels. Even if your gun does not explode, you will be beating the gun to death and end its life prematurely
“I know that you believe you understand what you think I said, but I'm not sure you realize that what you heard is not what I meant.” - Robert McCloskey