Glad to hear that you are making progress. Its all good, and its all fun. I wouldn't worry about the tank on CG just yet. Probably 90% of us are flying just fine with the tank in stock location. Yes, the DS821 (70 OZIN) will be plenty for the elevator halves. My Hitec HS225's on each elevator have the 50 ozin torque, just above a standard servo in a mini package and do just fine. I used them, to save some tail weight, and package the servos back to back in that narrow fuselage. If you go to a servo torque calculation page I'm sure that the huge rudder and big ailerons all need the extra torque. Elevator? not so much torque load, especially with one for each half- which is like having one HUGE servo for the elevator.