Charlie,
As I previously advised you in a thread on this board, the SAAMI mean (average) working pressure of the 12 gauge 3.5" is 14,000 PSI. The same of the 2 & 3/4" and 3" 12 gauge is 11,500. This isn't a matter of simple lengthening the chambers. One must ensure that an increase in pressure of 18% will not rupture the barrels especially since most barrels taper after the chamber. Cutting an extra 1/2 inch of chamber and forcing cone may produce dangerously thin barrels where pressure is very near the peak. I don't know about you buy I like all ten of my fingers!
One needs to ensure that the minimum barrel wall thickness (MBWT) of the entire chamber and for several if not more inches past the chamber meet the Hoop Stress calculations for the specific metal of which the barrel is made.
In simple terms, please DO NOT do this to Parker or other vintage SxS unless you start with something like a 12 gauge built on a #5 Frame with sewer pipes for barrels! Even then, I'd want to do the math first, or better have a professional do the math because those calculations are not simple (unless you work in the pressure vessel (tank) industry).
The CSMC Model 21 is available in 3.5" 12 gauge but Tony G. has the resources to ensure his special order guns such as those will be safe!
Please be careful Charlie.
Mark