I don't know the actual details or numbers (and too lazy to dig up) but I think that 340 family would be all over bit uneconomical for "hard cargo"! This must be true because otherwise there would already be a freighter version of it! Maybe for integrators (FedEx etc.) it would make sense but not for "real" cargo carriers.
Comparing to MD11 it has an extra engine and comparing to 747 it does not have the capacity, although being more economical.
I'm sure at least FedEx will have some 340-200 convertions in few years!?
It's same thing with 380 - good for someone who flies only relatively light cargo (integrators again) but questionable for "hard" cargo carriers!?
I like driving backwards in the fog cause it doesn't remind me of anything - Chris Cornell