Bill, that tank is from a Nighthawk bike, I think. The 80 and 81/82 650's, whether standard or custom, had pretty much the same tank but the petcock sizing for the fuel line is different on the 80. The 79 is completely different from all the other years too.
This is the 79 look... definitely boxier overall.
This is the 80-82 regular 650 and 650c... getting curvier.
This is the 650sc from 82-85... whole different bike and has the side attachment hole for the side covers.
I think the tank looks good on there but I see why you would want to change it. I'm going in the direction of a brat bike on our next project 80 650c and think that tank would look good on something like that. Do the holes line up with the current tank mount? Is there a way to get rid of the side cover hole area by cutting it off or maybe cover it up somehow? You're giving me ideas, Bill.
I like the original tanks too. We are going to keep the one off the 2nd bike for Kit to change colors on hers. Then on a whim, she can just go from black to whatever she decides to paint it and have a different bike.