1.how closely did you look at the intake gasket on the #8 cyl. area??
was it in good condition?? if not water could have got in thru the intake port...
2.have you looked closely the #8 cylinder bore specifically where the top of cylinder is close to the head bolts and/or the water passages??
there could be a hair line crack between the cylinder bore and the bolt holes or water passages;
it the area is dry you could spray some white or yellow paint on the area to help a crack appear...
3.what heads are you running and did we have any work done to them prior to using on this engine??
like oversize valves; hardened seats; valve guides; any porting??? this would be another source of water...