water in oil after heads swapped

The odd thing is that I am an old school Mopar guy with big and small block experiences. I do not remember ever seeing a corroded timing cover in the block passages. The thing is, in those days it was common to run water only. The blocks got nasty but I don't remember any corrosion. The only thing I can figure is that in 1985, a 318 would only be about 15 plus years old where this truck is an '89 which makes it 25years old. Anyway, 3500 miles later and life is good. Compression went up an average of 30psi with the new heads, runs smooth, and got around 14.5mpg overall towing a Dart wagon on a tow dolly from Ca. to Fla. Not too shabby. Just wish I was smart enough to have gotten the 308 heads instead of the 302's since I had to pay the cores.