The program I have gave a different answer than you for the 80 x 80 x 3mm box section. I make it 1.37 e6!! [1370000]
Ok found it. It looks like a 'times by 4' has crept in at the begining of the equation. If you remove it the answer becomes 1.37e6 and the torsional resistance goes up on the graph

Do you mind if I add to it or is a work in progress yourself?

Cheers
Ross