How does DaFont stand on fonts based on a public domain design? I.e. based on a typeface designed by an artist or designer who died more than 70 years ago.

Can I just click "yes, I created the font myself" radio button and attribute the original author in the description?

Yes you can, especially if you mention the original author.

(but it doesn't mean that your font will be validated... in case there is a technical problem or whatever)