95 Silverado w/350 TBI small coolant leak that I can't find

My small engines class put rebuilt heads on this vehicle this winter as a bigger project to help get them interested in auto mechanics. It had previously blown a head gasket between #5 and #7. 170,000 miles on engine. On startup, it misses(one cylinder) for about 3-5 seconds and then clears. Has done so since the first start after rebuild. No overheat, no exterior leaks that I can find or smell. Uses maybe 1 ounce every hundred miles or so. I pressure tested the system and after 2 minutes, the needle moved down slightly. Cap is new and no leak with test. There is no visible contamination of engine oil, nor coolant. Heater core was leaking so replaced with new one. We completely flushed the system as it was badly contaminated before replacement.

Honestly, if this is as bad as it gets, I can live with adding a little coolant. If it was leaking into engine oil, I should see or smell, or taste it(yes, i tasted to see if sweet). If it was head gasket, I should see it getting worse, right? My thought was perhaps one of the head bolts was weeping(all new bolts with sealer pre-applied).

I have worked the truck by pulling a boat with it to push the engine just a bit under load. Still no change. It doesn’t get worse, but still seems to have a small leak. I have also checked those push in heater lines at the water pump and at the back of the engine. No leakage.

Was thinking of throwing a can of stop leak in. Any hidden areas I should look at? There is not one drop of moisture around this engine. It doesn’t leak any kind of fluid anywhere. No wetness in the engine bay or by radiator, either.

Thanks for any and all ideas.

Brad