Understand Go proxy failures when routing

I have a simple Go proxy like so. I want to proxy requests through to it and modify the responses of certain websites. These websites run over TLS, but my proxy is just a local server.

func main() {
	target, _ := url.Parse("https://www.google.com")

	proxy := httputil.NewSingleHostReverseProxy(target)
	proxy.ModifyResponse = rewriteBody

	http.Handle("/", proxy)
	http.ListenAndServe(":80", proxy)
}

Result: 404 error as shown in the following screenshot:

From my understanding, the proxy server would initiate the request and close the request, then return the response after modification. I'm not sure what would fail here. Am I missing something w.r.t to forwarding headers to where this request is failing?

Edit

I've gotten the routing working. Originally, I was interested in modifying the response, but not seeing any change except I see the Magical header.

func modifyResponse() func(*http.Response) error {
	return func(resp *http.Response) error {
		resp.Header.Set("X-Proxy", "Magical")

		b, _ := ioutil.ReadAll(resp.Body)
		b = bytes.Replace(b, []byte("About"), []byte("Modified String Test"), -1) // replace html

		body := ioutil.NopCloser(bytes.NewReader(b))
		resp.Body = body
		resp.ContentLength = int64(len(b))
		resp.Header.Set("Content-Length", strconv.Itoa(len(b)))
		resp.Body.Close()
		return nil
	}
}

func main() {
	target, _ := url.Parse("https://www.google.com")

	proxy := httputil.NewSingleHostReverseProxy(target)
	director := proxy.Director
	proxy.Director = func(r *http.Request) {
		director(r)
		r.Host = r.URL.Hostname()
	}
	proxy.ModifyResponse = modifyResponse()

	http.Handle("/", proxy)
	http.ListenAndServe(":80", proxy)
}

The critical problem is mentioned in the documentation, but it's not clear from the documentation how to deal with it exactly:

> NewSingleHostReverseProxy does not rewrite the Host header. To rewrite
> Host headers, use ReverseProxy directly with a custom Director policy.

https://pkg.go.dev/net/http/httputil#NewSingleHostReverseProxy

You don't have the use ReverseProxy directly. You can still use NewSingleHostReverseProxy and adapt the Director function like this:

func main() {
    target, _ := url.Parse("https://www.google.com")

    proxy := httputil.NewSingleHostReverseProxy(target)
    director := proxy.Director
    proxy.Director = func(r *http.Request) {
            director(r)
            r.Host = r.URL.Hostname() // Adjust Host
    }
    http.Handle("/", proxy)
    http.ListenAndServe(":80", proxy)
}