Help finding powerful slim servo


So I started on a project with a normal sized servo but I found that I need another one, a better suited one. And I'm not quite sure about what is available and/or other possibilities for driving a heavy rail horizontally a few centimeters. The turnigy servo is a close match for what I'm looking for, the servo must have a metal casing and gears, around 60-90 rpm, as high torque as possible, has to be able to connect with gears, analog or digital doesn't matter (I think), must be full rotation, and it can not be higher than 10mm after it has been mounted. Easy right! :P I've seen it's possible to covert regular servos to continuous rotation, but I'm a novice and could easily ruin the servo if it's not as easy as it seems. Does anyone have any suggestions?

You may wish to post this question on RC Hobby forums where people are more expert on Servo models.