Disclaimer: While not an attorney, I did stay at a Holiday Inn Express last year.
This Youtuber is considered a business, and as such, has to ask permission for, and most likely pay a royalty or license fee to the copyright holder. Without that, depending on the strength of the copyright and the state in which the offender lives in, they risk the confiscation and destruction of the products as a whole so that the copyright holder can further protect their interests.
Now, with that said, perhaps this Youtuber should just rebody with stock 1967/68 Mustang body panels. he can then still build a semi unique automobile, all while continuing to thrive on his Youtube channel.
Bill