I have done this sort of thing before and I highly recommend you brace it as well before you cut it. As far as cutting, another method would be using a sawzall. Also, one thing you can think about is not worrying about making these long cuts perfect by running a moulding of some sort (like a threshold moulding available at the hardware store) along the edge, OR sink the edge into the wall by sliding it up against the wall when you are done cutting, then run a line along the edge with a pen then cut along that line removing the drywall behind it and replace it with something thinner or nothing at all depending on what you can put on the inside of the "car". Or simply just run a wood moulding or something on the wall along that line so that it fits up with that edge you cut covered. Painting a mural on the inside of with a view out the "windows". I just thought of that, cool.
Anyway you could still cut with accuracy and not worry about this idea of cutting the wall or something. But if you cut it and it looks good, you may not need to do this. I have never done this exact project but this stuff is coming to my head.
But running a saw across the middle of the hood trunk and roof that I have done and you do need t add bracing first. I cut up a 60 Dodge once for a stereo and adding those braces thru the hood would have been nice.