Speed Weasel
As airbus2boeing states, the 747-400F carries a much greater payload than the passenger version. With a payload of 128,000kgs and a ZFW of 288,000kgs, the MTOW is under 368,000kgs and so even the CWT is unused. You would need a payload of about 80,000kgs before a stab tank would be able to be used, and this is bleow the average freighter load (from my limited experience).
Airclues