I have my GTB mounted in my B4 just fine. What motor are you using? I use a 7.5 and a 13.5 and do not run the fan, no heat issues at all. If a fan is needed novak sells a 25mm small black fan that you would mount in front of the esc.
If geared correctly, 2WD buggy is very easy on electronics when compared to 4WD or truck. I have a B4 geared with a 84T spur and 19T pinion with my 6.5 Novak GTB setup. I don't run the fan and I've never thermaled or really gotten the esc or motor too hot.
also the J concepts bodies works better to run the fan. the motor size doesn't matter to where the esc goes. otherwise you can cut the stock body if you want to allow for the fan, not recommending that though. the smaller fans or less spacing would be better.