Tony is quite correct. there really is no benefit in using Tamiya gear boxes in an Heng Long tank. There are  a few very good 3:1 gear boxes that will drop right in. Even the Tamiya boxes require a drop down gear mechanism to gain proper torque and they still need either different pinion gears or motors to get back to scale speed. try the Asiatam or RCCommand steel geared 3: 1 boxes. Asiatam does not have steel but brass gears. These sometimes require new mounting stand offs, that is why i prefer the RCCommand steel geared boxes. They are a true drop in box.