Bolts or studs in the block for the head mounting?
Which ever one you put in, did you seal the threads?
As the engine expands with heat, the threads will allow coolant to escape into the rocker cover area.
Curt C made some good points. Since the system has not lost pressure sitting in the cool, I think I would reseal all of the head fastners. Use ARP thread seal, GM liquid teflon or Quick thread seal.
Reminder: If you do this, drain the cooling system first or
antifreeze will come out the bolt hole(s).
Hope this helps.