Drop a zinc anode in the block or one of those cheap-o JC Whitney ones that hang in the neck of the radiator and the problem will go away. The electrolysis of the dissimilarity of the metals i.e, steel on aluminum will cause that. Aside from the fact that those chrome necks are paper thin to boot.

It looks like electrolysis. This is common in ships. and engines. I had overhaul the engine in my front end loader awhile back because of this. I was getting water in the oil. When I tore the engine down there was a hole eaten in the sleeve that started from the water jacket and exited in the lower part of the cyl. bore. Here is the deffinition of electrolysis.



electrolysis
The process of decomposing an electrically conducting compound by the passage of an electric current. For the passage of an electric current two conductors called electrodes are dipped into a solution or melt of the compound. At the negative electrode (the cathode), metal ions and hydrogen gas collect, while at the positive electrode (the anode) negative ions collect. At the cathode metal ions gain electrons to become metal atoms and are deposited on the cathode; this reduction process has important industrial applications in electroplating and for reducing the ore of an impure metal to the metal itself.

I used to have a chrome thermostat housing on my 454 in my 70 Chevelle, I was tuning the engine and had managed to get the engine kinda hot,I had left the fan shroud off and decieded now would be a good time to put it back on as I pushed the shroud down it caught on the upper hose and with way to little effort the
upper hose slipped right off of the housing and proceeded to steam my arm like a crab,that was the last chrome housing I'll ever own, I guess it could also happen with a cast housing but not that easily.
As far as gaskets for the housing I always make my own out of thin cardboard,the kind like gaskets come in, it's very simple just hold the cardboard over the housing and either trace the outside with a sharpie or just cut it with scissors, pressing down on the card board to hold it against the housing you can cut the inside out with a knife, and for the bolt holes make an impression with the bolts and then cut them out also,It works great I haven't had one leak yet, and all the money saved on buying gasket's can be donated to the board :) MPB