In the uk, there are very stringent rules about almost every aspect of road vehicles that are not commercially approved.
I am guessing you would need to adhere to the same rules as a kit car.
In the uk, you need to take an SVA test as well as the normal MOT. The sva governs a lot of things (far too many to list) from things like seatbelt mountings to radii on any external trim or bodywork.
New Jersey? In the U.S.? You probably have a similar sort of test to do.
Contact your transport authority (DVLA for the uk).