Blown head gasket or cracked head would be the most common source of combustion gas in the coolant. A cracked or pitted cylinder wall would likely drive oil into the coolant too so if the coolant is clean, not a likely source.

I didn't think a domed piston worked at all with a closed chamber head.

I think you'd be better off to find a piston with a CH that puts it as close to zero deck as possible and machine the block to square and zero it. .039 head gasket would set your quench perfectly.

Sounds like you're keeping the stock rods so you might even find a flatop hypereutectic cast piston that would accomplish that economically.

Kevin