These pictures show what we have determined to be the real cause of our problem.
Notice how the header lip is extended well past the original pinch weld.
The little 'tabs' we figure are for aligning the header with the pinch weld. We aligned the header with roof inside the cab, not regarding this edge.
This just wasn't allowing the gasket to seat and fit up far enough on the pinch weld.
After much debate, I busted out the Dewalt angle grinder. Mike couldn't believe I'd do this, but I didn't see we had a choice.
We covered everything up and I sat on the dash, facing the cab and went at it.
After about an 1/2 hour, we cleaned everything up.
As you can see, this makes sense to our problem.