- Is there is reason why you don't have Node listen to 80 or 8080 ?
- Does the DNS redirection work ? -> Can you access your site with www.yourdomainname.com:1234 ? -> If no, then check your dns config, you should not have to deal with port routing at that level.
- Are you using Linux as your EC2 instance ? If yes, reroute port 80 toward port 1234 as explained here (http://www.cyberciti.biz/faq/linux-port-redirection-with-iptables/).
That how I do it, hope that helps.