You have to plan the job and how it's going to be put together. Some of it may be partially welded and pulled out of the car to finish. And/or it may be dropped through the floor. Or, like a lot of cars with funny car cages, etc., cut the roof off.

The chassis serves two purposes. Protect the driver and be the main structure of the car supporting the suspension with the desired rigidity.

The rulebook spells out minimum requirements. They are seldom sufficient to do both jobs well without adding to it and improving on it.