[Gpa113]

Here are some photos of the generic Scalextric Formula 1 / Formula 3 chassis's. 

Note how little there is to start with, how they can be narrowed very easily, and that you can shorten, or even cut them into 2 separate pieces. 
Cutting a chassis to shorten it is really pretty straightforward.   As Mr M has already suggested, cut , shorten, brace with plasticard or similar.  Seems a silly idea the first time you go to do it, but you soon get the hang of it.  Best to start with a cheap chassis,  and don't do what I did, which was to use a junior hacksaw.  I got away with it,  but a razor saw is the proper tool for the job.  Pendle's sell these as well!!

    bottom chassis standard,  top chassis cut and shortened. 

    standard chassis. So easy to trim.
    alongside one of the Majorette Hot Rods 


    chassis cut into 2 pieces, and mounted onto a plastic chassis Mater truck from Cars. Not my work, but a very clever way of adapting a simple chassis.