Cough has it right - the max fuel capacity (and hence range) is not much different between the -8F and the -8I. The Intercontinental has stab fuel that isn't used on the Freighter, but that's only a few hundred miles of range (whenever they turn a passenger 747-400 into a freighter, they remove the stab fuel system since it'll never be used). The difference in fuel burn per mile between the passenger and freighter at the same aircraft weight is pretty much insignificant.
The world cargo market is built around a system with leg lengths of less than 4,000 miles, and a 747-8F at max ZFW can carry over 150 tons of cargo using that system. For the most part cargo operators would rather carry more cargo and make a stop for gas somewhere rather than to make a long non-stop with a lesser cargo load.