The only thing that stands out to me is the #4 / #5 exhaust port in one of your pictures. It doesn't look like the others, white and dry, yet the pistons are equally colored. Water / coolant usually "steam cleans" the tops of the pistons. Could be it wasn't run long enough to get a good cleaning.
If it were mine, I would install new gasket and go again, flush out the old coolant and run water for a while just to flush out remaining gunk. The stop leak could have been added when you had the truck back at the shop for checking. With out being there or knowing the shop first hand, its hard to tell what they did.
Joe
