[zipper]

Modern, efficient engines won't fit on a 707 (they are too fat).

That's not correct. The KC-135R is the military version of the 707 and has high-bypass retrofitted engines.

[3niner]

My statement was overly brief. I was referring to actual 707s, the last of which was built in 1978. There have been many derivatives of the 707, and the 707 itself was actually derived from an earlier, military model.

The original 707s did not have enough ground clearance for modern engines, when allowing for the possibility of a hard landing. Newer derivatives, using newer engines, all had engineering modifications which adjusted for this.