For such an old fitting at your radiator, use only a flare-nut wrench to avoid risking the nut. And use a good amount of PB-Blaster (a rust release spray) over a few hours ahead of time.

If you're not familiar with what a flare-nut wrench is, imagine a 6-point box wrench (don't ever even think of 12-point wrenches for any work) that goes all the way around a nut, but with a small section cut way (to pass a line through it) so that there is still just a little portion of wrench that grasps the fourth and fifth lands of the nut on both sides. The advantage is that the wrench is grasping (let's say) 4-1/2 of the six lands, instead of maybe two of them with a traditional crescent wrench. A flare-nut is far less likely to slip and round the nut's high points.
You can get a flare-nut wrench of the right size at most automotive parts stores, and some big-box stores in the tools department.

And if you think you need more room to rotate the wrench, you can even buy a "crow foot" socket in a flare-nut style -- this allows you to put a socket extension on an offset flare nut end of a wrench, so you can still grasp the nut securely, but you turn it with a socket handle some distance away. This makes working in tight corners easier.

Below is a photo of two of my crowfoot sockets in flare-nut style: