Like 411A says, it's more to do with the fuselage length. Only a very few short body DC-8's were converted, most were -61's and -63's , which was where you could make the big savings (40% less fuel burn, wasn't it???) The 707 was never available with that lovely long fuselage.
Basically, it gave you a 757 without having to spend 757 money. One 707 did get converted (they called it the -700), last I heard the Moroccan Govt had it.