Coolant Flowing Out of Passenger Exhaust
Theory, flowcooler water pump blew exhaust gasket.
Are you saying that the water pump produces enough pressure to blow out the exhaust manifold gasket (how would that be possible)? OR do you believe the head gasket failed near an exhaust valve?
Personally I don't see how any water pump could develop that much pressure. Maybe the cooling system is not properly vented and the internal pressure got WAY high but I would expect hoses / rad core to be bursting long before the head gasket.
Water in the exhaust would be a blown head gasket from incorrect install, warped damaged head or deck surface, bad machine job on the sealing surfaces, bad head gasket, cracked cyl wall, pinhole in the head or cyl wall between the water jacket & combustion chamber. Are you sure its not a leaking freeze plug that is spraying on the manifold under the right conditions and trickling down to the low point (o2 sensor area) and then to the ground.
Not to mention if you had a blown head gasket there would be coolant in one or more of the cylinders.
To me its just a coincidence, nothing to do with what you did to the cooling system.
JUST my 2 cents