Plastic for cooling sucks.

I have a few original /6 aluminum thermostat housings sitting here. All pitted in the gasket seal area. They might seep but wouldn't blow out if they were to be used. I found a screaming deal like a year and a half ago on some OLD OLD stock TRW cast iron replacement thermostat housings for the /6. I bought a couple, on or my motor and one for future that's on the shelf at the moment.
Another spot I've had issue with pitting and seepage is the timing cover on small blocks. Again, I never had one explode and rupture all at once.id hate to think how much more of a problem they would have been if they had been plastic?