OUCH!. That's a lot of coolant.
I think you can stop looking for leaks, unless you're smelling coolant under hood.
The good news is you've got an iron block so head bolt threads are OK. Bad news is M52 heads are known to crack on overheat. It would be useful to find out which cylinder is leaking coolant, if possible, before pulling the head. This will help with head inspection. I'd pull plugs and pressurize cooling system, and see which cylinder fills with coolant. HF has a coolant system pressure tester for ~$60. Keep us posted.