[Porchdog]

Big pipe wrenches will get that loose. I'm used to having 36"s on the truck. There are two 60" at the shop. Pipe cheaters and come-a-longs as necessary.

That fitting looks more like forged than cast. When they are cast you can break them with a big hammer.

Some of the old buildings around here have 6" and 8" threaded fittings. You can't always demo those out, so you need to get them apart - if only to put a threaded flange or flange to groove adaptor on them.

If you can do it safely, oxy-acetylene on that fitting will loosen it up. The fitting needs to glow.
There could be hard-setting dope on the threads, which makes things more interesting, but heat will help with that as well.

Aluminum pipe wrenches are nice, but don't use pipe cheaters on the handles.