How is the car not running straight? Is it always pulling one way, or is it erratic and wandering either way? Is it only at high speed, or even while cruising around at partial throttle?
TC3s and TC4s were notorious for torque steer; if this is happening while accelerating and braking, then that's likely your problem and you'll mostly just have to learn how to drive it. Usually it will turn one way on acceleration and the other during braking. If the only thing you changed was the body and it was running true before, or if it runs true with a different body (try an old one) then there could be some asymmetrical aerodynamic issues with the body itself. If it feels skittish, has significant understeer at speed and wanders randomly then it's possible you have too much rear downforce and the front wheels are getting light. I wouldn't cut those holes suggested as it will weaken the body and cause drag. I would trim the rear "wing" (whale-tail) down.