Highly recommend them for steel tube headers but zero experience with any cast iron header.
Have had a set on my SBF in a '70 Mach1 for years now with Stage II's SS locking bolts. Never need re-tightening, no leaks, are easily removed from aluminum heads. Two things: always use anti-seize on any header bolts and especially SS bolts in aluminum. The other is the quality the welds on the flange facing the head. I've noticed that on the Hooker Comps I have and other makes, the weldments are pretty rough and need to be filed down a tad to ensure the faces are flat and won't leak regardless of the gasket material one chooses.