An IB-3 would be a better bet than the IB-2 for your application as you don't need the IB-2's extra complexity. That said, the Radio Lines Box will provide power to the harness, too.
The external tank telephone is usually connected by yet another box, the TAB (Telephone Adaptor Box).
There's a complete list of the boxes with explanations here: http://daimlerferret.co.uk/wp-content/uploads/2017/04/Clansman-harness-manual.pdf
Andy