Changing the response object from OWIN Middleware

My OWIN middleware is like this. (Framework is ASP.NET Web API).

public class MyMiddleware : OwinMiddleware
{
    public MyMiddleware(OwinMiddleware next) : base(next) { }

    public override async Task Invoke(OwinRequest request, OwinResponse response)
    {
        var header = request.GetHeader("X-Whatever-Header");

        await Next.Invoke(request, response);

        response.SetHeader("X-MyResponse-Header", "Some Value");
        response.StatusCode = 403;

    }
}

Questions:

1. Is it the recommended practice to derive from OwinMiddleware? I see that in Katana source, some of the middleware classes derive from OwinMiddleware and some do not.

2. I can see the request headers okay. Setting response header or status code after Next.Invoke in my middleware has no effect on the response returned to the client. But if I set the response header or status before the Next.Invoke call, the response with headers and the status that I set is returned to the client. What is the right way of setting these?