To help you (I hope ^^) :
I've got a 360 connected to lan. And the only thing I had to do to make XboxLive! work is to set up a nat on the following ports :
3074 TCP -> 3074 TCP
3074 UDP -> 3074 UDP (and not 2074 like you said ;) )
88 UDP -> 88 UDP
(From WAN, TO the 360)
Sometimes when I'm running the Live! test on my xbox, it tell me that its Strict, but often it tell me that its OPEN.
So I suppose that its OPEN in all cases and its just a bad detection from the 360 :)
And don't forget to make the rules to allow this traffic in your firewall ;)