yeah boy, it needs a good soaking for sure. After a good soaking I'd use a box-end wrench on the big nut and a "line wrench" on the smaller fitting to get them loosened. Are you putting on new lines? You'll probably have to as I suspect the little line going into the big nut will round off and you'll have to get nasty with a BIG pair of vice grips.........then again, it may break fairly easy after a good soaking with Kroil.
....as rusted as the little line is the nut may not turn on the rusty line and twist into?
...boy, aren't I full of good news today?! You can do it Woog, or bring it on up to see us and I'll help you. Hot dogs and Pepsi waiting.