Six wide and only one minifigure in the centre seems to be the way to go to get a decent looking minifig scale car. Going to 8-wide makes cars look awful.
I completely agree. It's difficult to get the right proportions on vehicles. The Mustang GT looks a bit too long, the Ecto-1 is just about right.
The Classic Batmobile is way, way too big, as is the police cruiser from the Kwik-E Mart set and family car from the Simpson House. All of them are fine for display, but they look wonky parked out front of a modular.