Looks like the DT manifold will outflow the Cummins by a good bit. Looking forward to pics of you pulling the head off as well as shortening the water pump. Maybe if shortening the pump doesn't work you could examine the possibility of making an adapter to put a commonly available but more compact pump on there.
I ended up having the water pump shortened just as much as Jon did in his build. It was a little less than what I had wanted, but I can make it work. The disadvantage is there probably isn't going to be enough room to run much of a fan... I think if I run Evans coolant it may help in the temperature department.