What is the minimum clearance I need between the truck cab and camper overhang?

In addition to the static clearance and frame flex, don't forget to also consider the movement that your tie-down system will permit.  You might even want to test this and see what happens in a panic stop situation.  I'm thinking you could use a piece of something like the crunchy FloraCraft FloraFom from Michaels.  Tape a small piece to the top of your truck and drive around.  If the cabover touches the foam it should leave some sort of witness mark from which you can better evaluate your dynamic clearance.