[RichardJ]

>>GM (Chevrolet) has produced high horsepower, high performance engines for years......every single one....came from the factory with manifolds...<<

Close, but not 100%.
The C4 Corvette L98 TPI engine used these tube headers, covered with the heat shields. The following LT1/LT4 engine had nearly identical cast manifolds and curiously covered again with tin shields.
Some believe Chevy decided the tube headers added too much engine noise.

>>Headers leak when you overtighten fasteners.<<

I found this to be especially true on my inline six. With the more even tightening ability with studs the length of the head, new gasket leaks are generally resolved by loosening the nuts.