I've seen the West Philly High School cars in person and I've chatted with several of the students who helps build it. This was in June 2010 at the Michigan Speedway for the X-Prize Knockout Round.
They used the chassis because they were available. I think the 160MPGe must be electric only. I know that the Ford Focus (their other car) at least used a Harley engine -- a sentimental favorite, but hardly capable of mileage even above 50MPG.
Since you are looking to build a from scratch chassis, why not go for all the efficiency you can? Aero drag is a huge contributor to energy loss, and it is a total loss, always.