In the Autumn issue of Air-Britain's Aviation World is a nice article about the XF-16XL project. Lots of photos too.
(footnote) Air-Britain used to have three quarterly magazines which you could subscribe too, Aviation World (which came as the standard magazine with a subscription), AeroMilitaria and Archive (which you could add either or both). As of the beginning of this year the three magazines have been amalgamated into one big (thicker) magazine, Aviation World