My advice would be to take the money you would spend on the Chamber Modifications and buy some proper 2 1/2 Shells.
If the old gun has very abrupt forcing cones you might consider having them relieved, more so to accomadate modern folded crimps and shot cups than anything else.
For the 16 I load 2 1/2 shells exclusively and I ain't giving up nothing for upland game or targets.
Rockie, if you can't hit birds or targets with 2 3/4" shells, you have a good point: you give up nothing going to 2 1/2".
(Sorry--the devil made me do it!)
Seriously, postoak's advice is very sound. About the only short-chambered guns that have problems with 2 3/4" hulls--assuming the pressure is appropriately low for the vintage gun in question--are ones with short and sharply-angled forcing cones. But if you're not going to shoot it a lot, just buy 2 1/2".
