You'll need to find something else to handle 1 1/4oz loads. I'd imagine this gun has 2 1/2" chambers.
I have a Cogswell and Harrison heavy 12 bore 1904 3" chambered, 32" barrels, I'd shoot 1 1/4 oz through it with handloads, not high brass US shells. I'd get a rem 870 for the swamp, not a nice gun like the E-grade Greener, but as they say, it's your gun and your money.....
Mike