How do I do this perfectly?

Hi alex, yeah those kind of areas can be pretty difficult.

Can you please post the .3dm model file of your object so I can test out some different possibilities?

It is always a good idea to post a model file along with a question like this because then it makes it possible to test things and also to examine your model in detail which is not possible to do with just a screenshot alone.

- Michael

I think you're a better off if you started differently...

Since I am naturally lazy, I always try to think of a way which gives me the most with the least amount of work ;-)

The attached test shows 2 curves, that is all that is used modeling-wise and a blend.



Extrude the large curve using "tapered", do the same with the smaller one (the cabin). Now take the cabin and scale it down as much as you want the blend from cabin to the car to be.
Take the original cabin curve and diff it from the body. You now have a whole in the body's top.
Move the cabin up as high as you want to blend. Now select the edges of the hole, the edges of the bottom of the cabin, and blend them.
Fillet the body and cabin to your liking and you're done.
Hope that helped...