If I'm reading you correctly, you assert that you can build a one off machine gun by filing an ATF Form 2, paying the Special (Occupational) Tax which is required by the NFA to be paid by a Federal firearms licensee engaged in the business of manufacturing, importing, or dealing in NFA firearms.
It appears that in order to correctly file the Form 2 you are going to need an FFL as a manufacturer of class III items.
In short you are going to become a licensed manufacturer of firearms to produce one gun which you can not sell? When you've invested that much you may as well open a business and go pro!
Take a look at form 1, it seems more to be what you need for this hypothetical one off project.
G