I doubt the tank from an '83 would fit the '82. But, I do know for a fact that all the body parts including the gas tank from an '81 CB650C will fit the '82.
When my '81 CB650C was rear-ended and the frame was bent, I bought a very crusty '82 CB650 Nighthawk. I transferred almost everything over to the Nighthawk. The only things that didn't swap right over were the front forks as the Nighthawk's forks had a larger diameter. I also had to rig up a way to attach the side panels as mounting points were different.
Anyways, the CB650C gas tank will fit your bike, however it does look different. But, if you happen to like to looks of the CB650C better, you could easily make your Nighthawk look exactly like one. It would probably cost very little to do a conversion after you sell off the leftover parts. Just another option to consider.