The exhaust stacks are steel and can be removed from the aluminum heads with the right tools. They are held in place with a friction fit and the exhaust clamps that attach the actual exhaust system insure that they cannot fall out when the engine is mounted to the airframe. All it takes is a properly sized pickle fork tool to provide outwards force while you tap it side to side with a hammer (the tool, not the stack). They are getting two coats of VHT Flameproof flat Silver that will be cure in the toaster oven. The very base of the stack is masked off so that no paint will be in the overlap with the head. You can see a flat side on the stacks and these have to be carefully clocked to allow room for the pushrod tubes.
In the second picture is a head all prepped and ready to start painting. The tops of the cylinders reuse the cardboard plugs from the case paint and the sides are taped over. In the middle the six white areas are paper towel plugs placed in the openings for the upper push rod tubes. The three studs that hold the exhaust clamps protrude into the foreground.
In the third picture the two heads have completed the paint process and the exhaust stacks are trial fitted. They will need to be driven in place with a block of wood and a heavy hammer prior to engine build. A dab of anti-seeze makes it easier and helps protect against dissimilier metal corrosian.