I feel your pain. What started as a simple pad replacement on a 240 turned into all new calipers, hoses and 6 new hard brake lines. I sprayed penetrating fluid for a week into the nuts, but they were completely fused.

I got some of the FCP copper lines for the short runs and they seem to work ok. After killing one of the longer lines, I ended going to a local brake and clutch shop to have custom lines fabricated. They used a product called easy bend (I think) which looks very similar to the copper/nickel original Volvo lines. Very easy to bend into position. If I did it again, I would've had them fabricate all the lines.

Bring your original fitting into the shop and they can match. It wasn't that expensive; 2x four foot lengths flared with the nuts was $35.

If you resign yourself to replacing the line, cut the line above the nut and slip a box wrench or socket onto the nut.