Ed, what revdocdrew led you to is good advise. Some people say it is alright to shoot 2 3/4" shells out of shorter chambers, but I believe in older guns the shell doesn't open all the way like you said and you could develop higher pressures than normal. In the web silte that he listed you will get lower pressure reloads, down to 4400 psi, which is great for the right chambered gun.
You can also still buy 2 1/2" shells by a few companies, mostly English, like Gamebore.
So I would stick to the shorter shells instead of low pressure shells in 2 3/4" length. IMO David

David