Argammon should try some slow custom cars and reconsider his opinion...
About the amount of work: yes, you can create new cars in a day - unless you want to give a really unique handling.
1, Search for the blueprint of the car.
2, Search for the detailed performance data (such as torque curve, air drag, mass, acceleration...), convert them to Stunts units and calculate the expected output (like PG-ability or air drag limited top speed).
3, Build the 3D shape in Stressed based on the blueprint (mind the paintjobs, the different car0/1/2 moduls, the limited [~6 cm] resolution of the game view, etc.)
4, Create a new dashboard with layers.
5, Edit the car's RES file to set the realistic performance of the car; the speedometer / rev meter / steering wheel / gear lever movements, etc.
6, Test your car - and go back to Stressed/CarBlaster to fix the unexpected bugs...
I think it's quite a lot of work.