Engine sits in there mid ship to the firewall, no or failing rear hood weatherstrip so water runs down over engine.
First suspect water point of entry, mismatched oil fill cap to aftermarket valve covers. Some caps only have a tin seal there, can get a new cap with a fiber gasket. Same with the PVC grommet, check that, have seen those grommets go into oil fill style holes in the valve covers, and that could easily leak water in.
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Process of Elimination, check all points of engine that could let water water in.
Get it all back together, park it where you are going to store it. With the dog house off drape a tarp over the engine. Pop the dog house back on and let it sit for a couple of weeks. Then quick pull of the drain plug, no water coming out then all is good. Then you know it was an outside water source getting into the engine. Pull the tarp and go for a drive.
