If you are not going to gain better brakes why are you doing this? Do you know anyone with a degree in mechanical engineering? They could probaly explain how the stresses on that hub are changing with the disk versus the stock drum.
Can you modify the hubs from the donor truck or make adapter sleeves to mount on the 6400's spindles? That might be a better way to approach this problem.
I absolutely agree with Grigg that your stock drums are better than a cobbled together disk setup.
