i would use the rtv gray high temp silicone, it will handle all the heat you want to throw at it with your cooling system.
it is used for flanges on transmissions, rear differentials and all over the cooling systems of heavy trucks with excellent results
i will stay just a bit pliable, meaning it will never crack, it bounds to any clean surface like iron, and is impervious to oil, fuel, water, antifreeze etc. and can handle several hundreds of degree's F. for years and years.
i have never seen it fail in my experience.
at 5-10 bucks a tube it is the sealant of choice in my books
the only thing i would not use it for is pipe fittings and for air fittings, it seems to blow out of the threads unless you wait for it to cure properly.
if you are concerned just use an o-ring between the flanges of the bulkhead fittings, problem permanently solved!
bob g