Don't get confused between Projects and Operations. What you are describing is actually akin to operational projects (albeit at the risk of bespoke designing and building each car).

Projects in the traditional sense produce new assets with new value with new benefits meant to be operationalized. You need to segregate associated costs so that you can depreciate the value of the asset once it's operational. Usually the cost is considered an investment and funded by separate finance.

Operational Projects support, extend, enhance existing business assets. This is the assembly line mentality and is more aligned with continuous improvement for which there is usually a fixed budget for the year and is completed during the normal course of business.

Yes there is some grey area but both are necessary.