I would be surprised if the position of caps would make a difference to how much coolant you got in. I too suggest the heater core; maybe an air lock so remove one of the heater pipes to see if coolant or air flows out.
One other thought: has the engine been left empty of coolant for few days? It is possible that the thermostat has rusted closed. That happened recently to a friend’s +8 when he was awaiting delivery of a new water pump. First time out, it boiled over within couple of miles from home.
