All of the above
Normal procedure says that anytime you change the thermostat a new 'O' ring should be installed. The old 'O' rings tend to turn into goo. If it leaks the cooling system cannot pressurize and the bike will always run hot.
You can feel a plugged radiator (kinda hard on a motorcycle to feel-- you need an infra red meter
http://tinyurl.com/y99udsbr) .
The bottom gets full of sediment and stays cool. I have seen this on old cars and trucks but not a C10.
I have seen radiator caps go bad because the spring gets weak and cannot hold pressure and boils over even before the fan comes on.
Sounds like what you have.
Also you need to test the radiator fan switch. (not the temp gauge sensor) I tested mine in boiling water like this video. You can test the fan first by bypassing the fan switch to make sure the fan works. The fan switch does go bad on our C10's due to age.
https://www.youtube.com/watch?v=lkwJoLBGr9QWhat was the temperature gauge showing when it boiled over?