the 8.4 would have to be regulated down to at least 6.5 volts. Futaba does not reccomend using more than 6volts on those servos, but
even a fully charged 5 cell reciever pack is over 6 volts when fully charged. I use LIFE reciever packs (6.6volt nominal voltage)
with all my servos standard and hi torque digitals and haven't had any problems.

An 8.4 volt pack may be too heavy, any 4.8-6 volt rx pack over 1000mah will be more than enough.