Bitter taste of defeat! So much coolant in the oil pan.

...........And the timing cover was taken out of the equation, at least from the water-pump side. The water did not reach the top of the thermostat hole - the leak is somewhere below the floor of the intake manifold as it now looks empty but liquid was still coming out.

The amount of water was the exact same flow - not pouring but not dripping either. It seems unlikely to me that after re-installing the timing cover and intake manifold, the exact same size leak would appear. So I think it must either be somewhere in a head, the head gasket, or the possibility that cylinder 1 got cracked somehow due to overtorquing a bolt that was too long or some such.

And now it's raining. Damn.

You have not eliminated the timing cover!!!! If there is ANY water in the block, the timing cover is a factor.

Have you got an air compressor? You can actually "after a fashion" pressure check it yourself.

Using the intake as a pattern, make 4 simple block off plates for the head water ports. You are going to have to drill and tap at least one of them for some sort of fitting to fill and to pressure

Make simple block off plates for the water pump holes in the front of the block.

Use thread sealer, like pipe dope or teflon tape and bolts, and seal off bolt holes in the timing cover.

If it still leaks, time to pull the head(s). Depending on what that brings, you can re-test it using your same block off plates.