Great post, thanks! I was hopeful the internet archive had an old snapshot of 7427466391.com, but, no such luck:

https://web.archive.org/web/20130815000000*/http://www.74274...

Does anyone know or recall what the even harder problem hosted on this website was?

Neat! Would have been a real head scratcher. Thanks for sharing.

The https://apod.nasa.gov/htmltest/gifcity/e.2mil URL still works, and now I'm really curious about why there are 2 million digits of e on APOD (Astronomy Picture of the Day)

I'd probably just look for the numbered IP addresses with these 10 digits that serve up NS or SOA records, and go from there.