I fit a set of these to my Torro KV2. The most difficult part was all the time filing inside the torro sprockets to fit onto the ETO shafts. Tamiya style shafts have a double flat vs single on the taigen/hl style if i remember correctly. In retrospect it may have been easier to grind the shafts instead of trying to mod the sprockets.

There is also an issue of the physical height - i needed to drill some new holes in the motor plate abd use some spacers to raise the gearbox, but otherwise was pretty lucky w/the KV. No major structural modification required.

I would recommend getting some measurements and pictures of the king tiger ETO box to see if it will work before purchasing.

But really it was all worth it. The ETO gearboxes are miles ahead of the standard type. For a big tank like the JT i'm sure they'd be amazing.