I have had very hot engine temps and no heat in the rad hoses on a tin top, back in my student days.
The header tank had frozen up, and I guess some of the hoses too, hence no flow around the rad.
Bearing in mind the temperatures we are seeing this could be the cause of the initial problem, after that yes you may have overfilled it and the excess 'Boiled out' of the cap, or you may have still had a blockage causing excess pressures.
Ensure the whole coolant sytem is warm, all hoses etc, and refill to the correct level. Give it another run and see how it goes from there.
Hope this helps
Doug