Sounds like you have driven the truck quite a bit since the first boil over event.
I suggest that you pull all 6 spark plugs and lay them side by side and look for a clean one. The clean one will have been steam cleaned.
That will tell you which cylinder is leaking water.
This test is as easy as as it gets.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission