That would be me..................
First question, is this the giant scale Byron AT-6 with the 101 inch wingspan? If so I find it hard to believe that "other control surfaces" only have a minimum of 48 oz/in. As a large model aircraft inspector that sets off many alarms under my cap.
One of the go to formulas in the AMA 520-a booklet for the LMA program is.............
The minimum torque requirement formula a conventional control surface is calculated as follows.
Minimum torque = A *Chord *Span * Chord/3 * Servo Arm/Control Arm
A = Airspeed factor (see Table 1)
Chord = average control surface chord (root chord + tip chord) / 2
Span = control surface span
Servo Arm: the distance from the center of the servo arm to the control linkage attachment.
Control Arm: the distance from the hinge line to the control linkage attachment
Table 1 states that a Propeller aircraft whose flight regime includes modest aerobatics, including loops, rolls, inverted flight and spins has an airspeed factor of 1.5.
Plugging in all of your information, if you use a 1 inch servo control arm and the control horn is 1.5 inches into the flap, would be (1.5 * 3 * 60 * 1 * (1/1.5) = 180 oz/in) .................. just a slight bit more than the 89 oz/in Futaba servo you currently have.
If it were me I would double check the surfaces in conjunction with the LMA formula and install servos accordingly. My KMP AT-6 recommends a minimum of 100 oz/in servos on the control surfaces .......... and it's wingspan is only 87".
I realize that the Byron At-6 is not over 55 pounds so servos of slightly less torque could be used. But with the availability of high torque servos being what it is these days (and not breaking the bank) why take the chance?
Cheers