I taped over all the passageways on the intake manifold itself.
Poured water into the opening for the radiator hose housing, no leakage into the carburetor part of the manifold, poured that out. Poured water into the opening for the carburetor, no leakage into the coolant passageways. So the intake manifold is intact.
The only thing I can think of that allowed coolant into the fuel/air passageways of the intake manifold would have to be poor sealing where open coolant passageways are next to open manifold passageways (two up front, two in the back). So I'm going to assume I installed the manifold gasket incorrectly.
If a head was cracked or a head gasket blown I would think the presentation would have been different, like white smoke billowing out the exhaust.
EDIT: It was a bit concerning to see such a difference in the look of the spark plugs, one side with all three looking black, the other side with all three looking white'ish. But after filling the intake manifold with water I now think I know why. I didn't know till then that the two sides of the manifold are separate. I poured water into one side but the other side remained dry. So in looking at the plugs I think it's now obvious that only one side of the manifold had the leakage (from the coolant mixed in with the fuel/air).
Now would that be the side with the dark plugs or white'ish looking plugs?
Either way, I'm gonna replace the whole gasket of coarse. I think I installed it dry last time with exception to the ends. Maybe this time around I'll use sealant around all the passageways.