Most likely the head gasket. At least you can start there. Pull a spark plug and blow air in look for bubbles in the coolant. If it is pushing coolant out the tail pipe like you say, it should push bubbles back into the coolant. May have to find top dead for each cylinder so valves are closed. Use a rubber tube inside the spark plug hole to feel when the piston is at on top so the valves are close. Than blow air into the cylinder for each. When you find the cylinder you now figured out part of the problem at least you don't have to do both sides.