Why is coolant leaking down the block?

This is tricky Steve and aggravating to say the least.
Some thoughts:

1. How good was the milling, grinding or machining of the cylinder head? Who really knows? Sometimes you get what you pay for.
2. The cylinder head bolts require a very specific bolt down torque. Double check that.
3. Was the gasket an OEM part or aftermarket? Makes a big diofference.
4. Finally, is it really the head gasket joint that's leaking? Look closely. Maybe it's the coolant sensor tap that's leaking.

Hi Steve:
Bud is leading you in the right direction.
My suggestion is to first track down the exact location of the leak.
To do this, first relieve any radiator pressure buy backing off the radiator cap (Cold engine). Then wipe down the area where the leak appears to be. Make sure it is dry, then check with a kleenex to look for the slightest bit of damp. Get a light and mirror so you can clearly see what will be happening. Start it up, run for a couple of minutes, shut down, look for the leak.
All of this is necessary to find the source.
It is also possible that there is a crack in the block or head, both of which are really annoying.
All the best.