Okay, so I was tinkering around my friend's shop some more today and realized a couple things that I didn't remember before.
The plenum bolts onto the ITB's, and the ITB's bolt onto another manifold, and that thin manifold bolts onto the head.
Now the piece that needs to be made, is the piece between the ITB's and the head. It's basically going to have to be a two sided adapter, one that works with the RB25 head, and the other face for the ITB's to bolt up to.
Also what makes this more complicated is the water ports that go between each pair of intake runners.