Happy to help, esp. another potential 16er
Just thought I'd add this (which is what Ted said):
The Winter 2001
Double Gun Journal has the article by Sherman Bell called "Finding Out for Myself" Part V "Long Shells in Short Chambers".
After extensive testing carried out in a very professional manner he concludes:
"With loads that are sensible in a light 21/2 inch gun, we see no dangerous pressure levels produced. I see no reason, related to safety, to modify an original 2 1/2 inch chambered gun to shoot 2 3/4 shells,
If The 2 3/4 Inch Load You Intend To Use Would Develop Pressure That Is Safe In That Gun, When Fired In A Standard Chamber! 
