Bozi Tatarevic writes:
Dallara is a world class racing company and they are very good at what they do but a stock car is a very specific design that needs specific engineering for stuff like yaw and over time it has become fairly apparent that engineers on the teams weren't brought into the mix enough.
Probably because Dallara isn't in the mix
Spec chassis
In contrast to the body-on-frame construction of previous generations, the Next-Gen car will use a
carbon fiber tub. This construction allows the driver to be positioned farther from the door panel of the car, further increasing its safety.
Dallara was initially rumored to be the exclusive supplier of the new chassis, but it was revealed that Technique recently opened a shop in Concord, North Carolina, indicating that they will manufacture the chassis for NASCAR teams
It looks like besides sticking an OEM nose and maybe a tail piece on the car it is pretty much done.
John Probst, NASCAR's senior vice president of racing innovation, commented that the "P3" prototype Byron tested was nearly 100 percent of the final product
en.wikipedia.org