Trust me, I feel your pain. Finding the right servo horn/arm is not always an easy task. RC platforms have various ways of connecting the steering linkage to the servo. Some steering linkages will mount above, some will mount below the servo arm. I've sometimes spent hours searching for (what I think will work) the correct arm to use. Different servos have their mounting ears located at different heights sometimes as well, and is why kits will come with spacers to raise/lower servo for best position.
Looking at photos of stock Sabertooth, looks like the steering linkage is mounting under the servo arm. Would need to find servo arm that sits high off of servo body, and that doesn't have that extra material under the arm.