The Remington "Game loads" in 16 gauge are cheap and effective. 2 1/2 dram eq. with an ounce of shot. The RST 2 1/2" is 2 3/4 dr eq and 1 oz of shot. The rem load actually kicks less.
I wouldn't lengthen the chambers. It will take the gun out of proof and lessen its' value unless you then have it re-proofed which is a terrible waste of money. Stick to the 1 oz, 2 1/2 dr eq loads and you will be fine. It's not the pressure that will hurt the gun if the barrels are of original thickness, it's the recoil.