The Slave Songs website has been linked many times in spirituals contained in the Permathread, and lyrics of these songs are already in Mudcat (see Spirituals Permathread mentioned by WYSIWYG). Other versions from various collections often are included with them in the individual threads on spirituals.
In thread 65605, Joe Offer asked for volunteers to make midis for the collection. I believe MMario is one (only one?) who answered the call.

To find the Spirituals Permathread, look in FAQ or use this link to 38686: Spirituals Permathread

Thread 65605: Request

I seem to remember that Allan's "Slave Songs" was in the "Links" but the list seems to be gone and I can't find any of the old links with the new, abbreviated fill in the blanks system.