Hi Bob,
I had a similar problem years ago with an o-ringed block and no grooves in the heads. It was fine n/a, but when I sprayed it, it would leak. It would pressurize the cooling system and fill the overflow every time. I ended up putting an o-ring in the head just outsied the o-ring on the block to make a S-shaped squeeze on the gasket. A local racer recommended this. Never had a problem since. I used yamabond as a sealer before and after. I used to use moroso or pioneer ceramic seal too, because they usually would have small leaks like you describe when you first fill it with water.

Joe