And after re-assembling and bolting down the head and cam, putting the crank at TDC, and pulling the forward side (the "pull" side) of the cam chain taught, I found this:
You can see that the witness marks on the sprocket do not align with the gasket mating surface. I'm pretty sure this means the cam chain is stretched. I posted this pic in the main XL600 thread for feedback, and I'm including it here just for continuity.
-Christian