Re: 1958 Lark 35

[Don]

Boy, this is a tough one. Impeller chunks could be stuck anywhere, including in the exhaust housing/water tubes. Do your best to check every passage possible while that head is off. What do you mean by "the channel of the head"? Is this a spot that could have stopped water flow?
I know you have probably answered this, but have you run the engine with the thermostat cover removed after the engine stops pumping/overheats? I keep asking this only because having a definite answer might help direct you/us to the problem area.
In other words, if no water is coming out of the thermostat cup when/after the engine overheats, that would direct us toward the exhaust cover/water tubes or areas before the thermostat. But, if there is plenty of water flow at the thermostat cup right after overheating, I would be looking for problems after the thermostat. I realize this testing scenario is imperfect, because the impeller chunk might dislodge after shut down giving inconclusive/false test results. But, if you see the water flow stop after pulling the cap, that will say something. You may be able to loosen the screws once the motor starts to overheat without stopping it, which might minimize false conclusions.
Hopefully Frank/Garry will weigh in more on this.
In any event, use a little gasket OMC/merc gasket sealer on both sides of the head gasket. Be sure to check the block and head for flatness as well.
Hold off on reinstalling the head until we have heard from others, sure don’t want you to be forced to pull the head again after you have reinstalled it.