93 Cobra 5.0 coolant leak

A friend has this 93 that has been in storage for 20 years. He wanted to get it running so, we replaced the fuel tank and FP, starter selenoid, new fuel injector o-rings and flushed the fuel lines. Thing started right up and runs good, but there's a coolant leak coming from rear of of engine, left side. We looked all around there with a mirror and can't tell for sure exactly where it's coming from, here's the question. We used the old upper plenum gasket with a dab of sealant (yeah bad idea)... but could the leak be coming from there/does coolant flow through the intake plenum? If not, it must be the lower intake gaskets or hopefully not, the head gaskets. Thanks for any ideas here.