The only thing that really should be needed is to make sure you accept the relevant ipv6-icmp packets:
ip6tables -I INPUT -p ipv6-icmp -j ACCEPT
Setting your connection to 1280 will work, but it is overkill. In IPv6 MTU actually is route dependant. Generally you send your connection to the largest MTU value supported by any route you will be using. Then you let the linux kernel use the ipv6-icmp messages to reduce the value per route, as needed.
I was having horrible IPv6 problems on my laptop, because it turns out the default Fedora 20 configuration does not have this firewall rule. As such it would send IPv6 packets that are too large and then wait forever. The typical symptom you will see when you have this problem is you can ping www.google.com, and even perform a wget, but you can't open google in your browser without disabling IPv6. Worse since it is route dependant, if you are using multihome or there are multiple routes to the website your are trying to retrieve sometimes the webpage will open, sometimes it won't.