That Sketchup model is pretty crude (for instance, it has no tumblehome or radii). If you look at real blueprints of it you'll see that the windshield to roof transition is curved well enough that you don't need that roof modification.
When you get down to the final planning and measuring stages, I would recommend using these accurate drawings to avoid surprises later on.
The-blueprints.com has a high quality vector version as well (under "various cars") but you have to pay a subscription fee to download it. It might be worth it if you're as serious as it appears.
Keep up the good work but don't forget that the car is a lot more complex than that model!