Good news - I heard back from Dartmouth and have their OK to use them as a source for the Longfellow translation of the Cantos!
The note follows:
"Dear Prof. Santoro, the fact is the Longfellow trans. is (or should be) out of copyright. It was first pub'd. nearly 150 years ago. Our 'pages' do not claim that his work is protected by copyright, only ours in making his text available. And you have done the right thing, which makes it all the more pleasing to say that, if you choose to employ this electronic version as your text, all you need do to have our permission is acknowledge the DDP as your source. Best of luck with your project, Robert Hollander, Director, DDP"