You can respond with a custom text, using the webhook response mode last node.
About changing the HTTP status code dynamically, that is sadly not possible. What you can do is always respond with a JSON that has two properties, code, and message. Then, the client would ignore the HTTP response status code and only use the code in the response body to determine what to do.
A community member created a future request for this feature. Make sure you go there and upvote it.