antifreeze leak

i have a 86 1500 with a 305 i was having a problem with the water leaking out about a year ago so i put a head job i had an extra set i had rebult and put on it and that seemed to solve the problem and then it started leaking again so i put another head job on it i had the origanal heads rebult and surfaced everything and put it back on and its still leaking the same amount of water as before so i would think that is was not the heads leaking in the first place but i can never see anywhere under the truck where it is leaking there is never a puddle so it is only leaking while running it has a new water pump and thermostat i was wondering if anyone has any ideas what it might be the only thing i can figure is the block has a little crack in it but it was not noticible when i put the heads on either time

My 86 seems to leak a little around the front of the intake manifold. Around the thermostat housing. But it is a really intermittent problem so I haven't worried about it.