LA 360 Internal Coolant Leak

How does the oil look? Look for moisture in the cylinders, but at this point, trying to check with a borescope is just dancing around the problem. You're gonna have to crack that puppy open regardless of what you think you may see. And the sooner the better, as TMM stated.
Bad gasket: gotta open it up.
Cracked head/cylinder: gotta open it up.
Get actual eyeballs on the problem. Pay close attention to the front of #1 cylinder bore with the piston at BDC.