I was a Chevy tech for 20 yrs...couple of areas to look.
There could be a connector at the rear of the intake manifold that connects a metal pipe from the manifold to the heater core. Those get corroded over time and can leak.
Also the intake manifold gaskets themselves can leak in that area too.
You can rent a coolant pressure tester from the parts store and pressurize the system to locate the leak.
Hope this helps