/3/ - 3DCG

I'm trying to make pic related.
and everything about it is easy (because it's a boxy van) EXCEPT for how to approach this cylindrical shape that flows through the side panelling.
The thing that's giving me a headache is the fact that it is slightly bigger before the front wheelwell and gets slimmed down before crossing the entire rest of the side.
I cannot for the life of me figure out how to replicate this accurately.
I've done other cars from this era, and older, it's pretty much all I've been doing recently. This is still just fucking with me so much.