The older (pre digital) GTO's are front motored and the later digital and Evolution digital upgradable ones are rear motored.
The later digital ones were made for having a chip in them so they are the easy way to go if you can get one. But they have their guides moved back from the front of the chassis. The older ones have the guide as far forward as possible, but with the motor in the front, the drive shaft that runs to the back and an interior that is deeper there is little room for a chip. You will have to trim the interior until you fit the chip above the drive shaft. With some cutting, soldering and some engineering it should not be to hard.
Randy
