Re: question on powder.
No, I would not say it's a safe call. I'd say it's dangerous. I don't know how tight your gun is, bu I had to reduce the 870 load in my gun 20 grains from the sierra manual. I'm glad to be here after that deal. I never got more than 75 grains of anything in mine and 75 of R25 blew P-Pockets out for me. That was a 140 AMax at 3400+ in a 29" barrel. Many never survived the trip to the destination with a 1:9 twist. Like 50%.
In my gun, I thought the 94gr load in the sierra manual was a tad liberal so I started at 84, 10 gr lower. I still keep that case on my reloading bench as a reminder of being careful. Ever seen a large primer pocket? I'm talkin 3/8". Mmmm BANG. The case was liquified down inside the bolt and the brass surrounded the ejector pin like foil. Holy big kabooms Batman!
Interesting how your gun is so different from mine. I'd still drop to low 70's for the 872 to start. Load 1 and work up in realistic increments. It won't take long to find a good spot. When it get's hot, it'll get hot in a big hurry.
If you have something that you disassemble and reassemble enough times, sooner or later, you'll have two!