I would agree that you are overfilling the radiator too. The colant should only go to the top of the fins within the radiator. That is about (guessing) 2" below the filler neck.
It is no wonder your truck is puking. The thermastat is opening up and then the water pump is pumping the hot coolant into the radiator and it has nowhere to go but out the overflow tube.
As for the hole in the stat 1/16 1/8 3/16. Anything is better then nothing.
