There are 2 considerations when using a 2.75 inch shell in a 2.5 inch chamber: pressure and recoil.
Using a low-pressure load, you should not experience excess pressure that would endanger the barrels, lockup or gun generally.
OTOH, using a 2.75 inch shell in a 2.5 inch chamber will result in a marked increase in recoil. It just does. That may not be good for the stock, and it may lead to other things working loose. If, as I suspect, your gun is a pre-war double that means the wood is >85 years old. Not just age, but also you have no way of knowing what the gun and stock have been subjected to in the intervening years. Probably a steady diet of higher-recoiling 2.75 inch shells for a good chunk of that time when 2.5 inchers were unobtainium. So, you might wind up trashing the stock while saving a few bucks on shells.
It's your gun and your choice, of course.
