First off, what is your experience level? I see that you have only a small handfull of posts here on RCU (not that it means anything) but you are considering using 54 oz/in servos on a Giant Scale P-51 ...that should send bells and alarms ringing to a lot of modelers.

What P-51 is it? That tid-bit of information wouold better help us help you.

Please do not us JR DS811 servos in a giant scale warbird. 54oz/in servos will not work on ANY flight surface. Throttle, choke, retract valve, yes, but not on the ailerons, elevators or rudder. Please check the torque requirements as indicated in the build manual for the airplane. You may be able to get away with the DS821's on ailerons, go with something 120 oz/in or better on elevator and 250+ oz/in on rudder.

Spend the money on quality servos unless you want a quality trash bag full of quality airplane.

Seriously, stronger and faster servos will be much cheeper in the long run than a crashed airplane.