I really don't think they did, the problems that need to be solved to retrofit existing airframes to carry a lightweight 300' load pale in comparison to what's needed to design a whole new jumbo sized airframe. Especially since once they've designed an airframe that's only good for carrying large low density loads to rough fields, then that will be the only thing it's good for.

A large wide body airliner with a big-ass shell and gravel kit retrofitted is still a large widebody airliner. Just one that happens to have a decent amount of headroom.