my guess is the end product will probably drive quite badly. Not a slam on the builder though.
My father's done 3 frame-offs. Rebuilding a professionally designed and manufactured car requires a hell of a lot of sorting, even for the most careful restorer, before the car is really driveable. It's always taken him at least another 40 hours of driving and tweaking before the car runs nearly as good as it looks.
Imagine how long the sorting process will take for a car that was engineered and manufactured as a one-off by an amateur in a basement. If his skill was really exceptional he won't need to do any frame fixes or substantial disassembly to get it safe and driveable.