I agree with Bro. David that chamber lengthening is not necessary. Forcing cone lengthening is not unreasonable, and will mitigate the rise in pressure when using 2 3/4" shells somewhat.
Interestingly, off the shelf modern 16g Fiocchi and Remington Game Load shells after firing are 2 9/16" so would be a good option, rather than the excellent RSTs, esp. after glasbedding the head of the stock.
I would limit your loads to 1 oz.; 7/8 oz. for targets would be even better.