It's not too bad. Some cars are easier than others. CLL lights will need to move to the side and sometimes you have to put a spacer on the rear mount. Spacer = stock nylon bushing. 6 cells also makes your center of gravity much higher. You can roll the car multiple times. I pulled my 6 cells because of that. How fun is it if you can't contrl it.?
i did the external mod.. very much glad i did. especially on fwd cars. does seem to twitch a bit though. you have to shave the sideskirt mounts on skyline