[olddog]

With very little in the radiator it sounds like there is a leak (not necessarily the only one) down low in the block.

Pull the plugs and see if it will turn - with a breaker bar - by hand. If it turns see which cylinders push out anti-freeze, and how much.

Spin it to clear the liquid and then do a compression test. If the compression is good, you may luck out.

Honestly, I have a real bad feeling on this. Kinda makes you sick. Hope I'm wrong.