Funny you should mention this. I was literally in the process of putting a driver in a WBB chassis (first photo below), which is cut maybe a scale 4 inches above the waist. You do what you have to do to get the figures to fit.
I don't think I have put any figure in any car without cutting or filing away something. The most drastic is the figure in the second photo, cut just below the neck, with just a trace of shoulder.
EDIT 2 hours later: I finished the project and I thought one other figure I was working on might interest you. The photo below shows a Bachmann taxi lightly modified. Goo gone removed the checker stripes along the sides and taxi logos, and a razor saw the roof. I had to make a cover over the motor - it's just a piece of thing styrene carefully cut to just sit on top of the motor and then covered with plastic electrical tape (matches the Bachman interior perfectly). Also I put electrical tape on the inside of the car to simulate upholstered doors inside. Styrene cut to shape for folded down top glued behind that piece and painted. Not much too it.
Notice the female figure I put in the front seat. I cut her to fit with a notch in her rear and trimmed her feet off 'cause they would not fit. The driver can be just a bust because he has the wheel hiding the fact he has no legs, etc. But be leaving so much of her she looks much better in the car.