LA 360 Internal Coolant Leak

Some other areas to check when you pressurize the cooling system that were not already mentioned above already
- 4 corners of the intake where head meets intake there are cooling passages in the corners
- Front timing cover and Water pump. Some of the bolts hit water and need to be sealed. Leak could also be around the water pump if you did not look there.
- Frost plugs on the head and block. Two of the frost plugs are on the back of the engine which are covered by the bolted on trans. Look for leaks or bubbles around the visible ones when pressurized.
- Thermostat (I assume you looked at this
- Any of your hoses and their connections
- Heater core inside the car.
- A crack in a head or block
- Check your oil for water as mention in the prior response. Any whitish froth in the breather caps, dipstick or under the valve covers? If yes could be coming from a few of these and the above mentioned items.

Good luck. I have had to chase these before.