You'll probably need to go to 3/4 oz. to find low pressure 20g loads.
Dave in AZ started an excellent thread on Shotgun World, and you could probably PM him
The good news is that some 20g hulls are shorter than 2 3/4"
If the head of the stock has been glasbedded, no worries about 1200 fps.
It is my opinion that lengthening the forcing cone is safer than lengthening a 2 1/2" 20g chamber. Because the external taper of the barrel is less acute than the angle of the cone, wall thickness is usually greater in the cone than the end of the chamber.
As Jent observed, being able to lengthen the chamber is wall thickness, and the expertise of the lengther, dependent
Please let us know what you come up with Dale