IIRC, Steve Zihn built that gun. He's also built a couple or so 4-bores, and a bunch of 8's.
And it is a 2-bore, not a 2" bore.
Top load for medium-bore PRB rifles used to be considered as powder weight = ball weight. I do not think I would want to go over 1000 gr of 1F in that thing, much less 3500 gr. 8) And by that I mean stand next to someone shooting it. My shoulder long ago advised me that the upper limit for me is 10ga breechloading shotguns, or something less than .375H&H cartridge guns
I shoot an 8-bore trap gun fairly frequently, and 1F produces a markedly softer shooting load than 2f, even bumping the powder charge to get your velocity close to the same. I don't shoot the 2-oz load in it very often.
A good 8-bore PRB will make any land animal on Earth DRT with a good hit, although a 6-bore might be a little added insurance.