This might be a long shot, but maybe one of you can help me.

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Not 100% sure anon, but it looks like they type of nuts I've seen on kitchen sinks. I've only had to deal with it a couple of times, but I used pic related to deal with them. Do a search for Do-all plumbers wrench. This one is made by Plumbest for ~$20-25ish.

Or just cut it off and replace.

hammer and chisel loose then unsrew by hand.

>>1155096
I did consider cutting it off because I don't plan on reusing the part any how. I'll take a look at that wrench. It doesn't look adjustable so I'm not sure if it will fit?

>>1155086
Use channel locks on it

>>1155103
It isn't. It's made for a specific kind of nut, the ones on the bottom of kitchen sinks. They are all the same size.

>>1155103
Also a strainer wrench would work and is open ended. That blue one I posted is for 2".

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>>1155086
Ever heard of a strap wrench? Pretty useful for those big square cleanout plugs too.

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>>1155401
this or any number of tools designed to remove a cars oil filter

the other slightly more dodgy method is to stick 2 robertson head screwdrivers in opposing holes on the bottom part then use something to twist them
you can also clamp the wood or whatever to make it a stronger piece so it doesn't shift

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>>1155086

Regular plumbing slip-joint pliers will work but I have pic related for jobs like this.

I see they're `$50 now but when I bought mine I paid ~$28 - money well spent.

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>>1155086
I see you have a Mansfield tank. Are you changing the Mansfield flush valve to a common flapper style?
If you aren't changing it and the inside gasket is still OK you shouldn't need to change the flush valve seat.
If it just leaks around the seal they are only a dollar or two at most hardware stores.

OP here. Thanks to everyone's ideas. Ended up going with a strap wrench >>1155401 and it worked like a charm.

Cheers.