My wife is an admin on the PG project. She puts in a lot of effort to keep the site secure. I think they welcome efforts like yours, and reuse is part of why they exist! They also research the copyrights, to make sure they are clear to share.
I use Project Gutenberg a lot personally, and in fact I'm using one of their books as a study for my upcoming startup:
https://github.com/carter-brothers/hand-propped#first-stage-...
One thing I really like about the PG site is that a few years ago they removed most of the JavaScript, which IMO makes it more usable. I haven't looked at Standard Ebooks yet, but I look forward to. Thanks!
One book I'd like to see on Standard Ebooks is "The Flying Girl", which my wife and I just read and loved. It's by L. Frank Baum (The Wizard of Oz), and is about a flying startup in 1911, which I previously posted on HN: