340 coolant trouble

If you burned the coating off the headers, that tells me you are running LEAN. Lean is HOT. I would richen the mixture, and your water and engine temps will go down.
The bubbles are air bubbles, and needs to happen. The bubbles are also a result of boiling, in the heads, probably near the exhaust valves. This is common, and once the majority of the air is out of the system, and the rad cap is on, the bubbling will be stopped by the pressure of the system.