Looking at your Power HD High-Torque Servo 1501MG, would it be safe to assume the provided dimensions are the overall dimensions of the servo?
If that is the case, would it be possible to provide the dimensions of the distance between the mounting holes in the X and Y direction? And also if possible, the location of the center of the servo horn in respect to the servo holes.
Power HD has some dimension diagrams of a standard size servo in this PDF on their site. The top three views, at least, seem to match up with the dimensions of the 1501MG, so the mounting holes are separated by 49.5 mm and 10 mm.
The distance from the shaft to the mounting holes is not specified, but from my measurements, it seems to be about 15 mm in the lengthwise direction from the closer pair of mounting holes.