You really want to get some sort of side exhaust motor as the GT was designed with side exhausts in mind, they tend to fit better.
As for Carbs, slide or rotary will work, but you will need to change the linkage from the stock setting to get the slide carb to work properly.
Other than that, sounds like you want something fairly mild in the way of horse power. I would really suggest asking the other gas racers at your track what they are running to give you an idea of what you want to get.
If I can't break it, no one can...