I have an NAD T762 and I am guessing that the setup is pretty similar to your unit. The T763 is only a 6.1 receiver, so without another power amp, 7.1 is impossible. If I understand you right, you are asking about the choice on the setup menu between "7.1" and "5.1/2.0". The way I read it, this choice is only worthwhile when using an external power amp to power extra channels or a second zone. If you wanted to use a separate amp to get a second zone, you would choose the "5.1/2.0" choice...giving you 5.1 in the main zone, and 2.0 in the second zone.
If you want 6.1, just specify this in the speaker setup menu. Set the "surround back" to "on" and that should be all you have to do. Don't worry about any of the other stuff unless you get another amp to power extra channels.