You mean, properly dressed, corrosion inhibited, and safely loaded, that bar would not perform as designed. There's no proper way to fix that bar that results in working the way it's supposed to.
When you damage a bar like that with a compressive force, the metal under the damage is compressed, which also means hardened. So, even if you smooth out the nicks, you have hardened metal in those areas where stress risers will still form. So you have to go deeper than just the nicks.
Then, you have a section of bar that's at a smaller diameter. The outer diameter of the bar is key in determining the spring rate of the bar. Just look at how much difference a few hundredths of an inch make on the rate of the bar.
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For the A-body bars you see .02" make a 10 lb/in difference in the rate. What will it take to clean up those nicks? Now you have a section of bar operating at a different effective rate than the rest of it. You think the overall rate of the bar is going to be the same? Do you think that the section providing less resistance than the rest of it will cause metal fatigue/stress in the area it necks down?
Sure, if you smooth it out enough you might not get a big crack to propagate right away from the end of one of those grooves, but eventually that weakened section of bar will come back and bite you. And really, it's already a 45+ year old spring with who knows how many miles, so who knows how long it'll last.
Toss 'em. Even in mint condition they're not worth that much.
