I have got to hand it to you, this is a very very cool truck you are building, I have long thought of exactally what you are doing. I have a 72 C-30 that I am working on right now. Somthing you might consider is installing a NV4500 as well, I will be posting pictures on a build thread here shortly, I think you might take a lot away from it.
Here is a pic of my Dually "The Boss Hoss"