I am not in the US, but where i am afaik anything that modifies the car like this would have to go through some sort of recertification process.
It would be perfectly legal on private property but not public roads
It would be perfectly legal on private property but not public roads
The modified rear door might be a problem, but where I am you could simply keep the original open, or detach it.
French, Portuguese and German rules of the road for example are all different in statutory strictness and enforcement strictness.