yes it goes between n3 and T. took me a while to figure out what you were trying to draw. (though you had 6 batteries for a while)..
I would just use the output bolt of the shunt as the T. just bolt both lugs to it.
if you plan to add more large neg wires later then put a big bus bar there instead.
on 2nd thought, where is your inverter
chassis ground currently going? normally it's that same bar as well. and will also be big. either 1/0 or 2/0. so you'd have 3 there and need a bus bar of some sort.