Sadly, passenger B747s are no longer economical so I am talking about adapting freighters. The C of G problem has already been addressed by systems used by aircraft spraying oil slicks...a simple computer programme to sequence the tanks, starting front and rear and working to the middle. The tanks containing the chemicals are double skinned and vented in case of leaks. A B747 on a 12 hour flight will use 120,000 Kg of Jet A. The same aircraft would use 40,000 Kg of hydrogen so there is an 80,000 Kg saving for the weight of the tanks.