If I read it correctly, the original design was for an AF cargo plane, actually. As the AF dropped the Boeing proposal, airlines asked for a larger plane, and Boeing converted the freight design into a passenger plane. So no wonder, you could so easily convert it into a cargo plane.
AFAIK, relatively little was shared between the proposed Boeing transport and the 747. Certainly there are influences: the cockpit and the nose door are the biggest ones. I think little else was carried over, given very different mission profiles.
(The winner of that competition was the C-5 Galaxy.)
