It would have to be a pretty thin transmission on the end of the motor. The Tiger has a lot of room, but it is limited. And I believe there are budget considerations in play here. The best bet is just to put in 4:1 gearboxes and call it a day. The "4:1" gear boxes, in that tank, will take either the 380 or 390 Motors.

Post some photos of the inside of your tiger with the gearboxes and Motors that you're using right now. I just might be able to help you.