[71GMC_3/4T]

You also need to make sure your exhaust system is sufficiently clamped so the header can't shake back and forth. There should be a factory hanger about 1' or so back from the collectors.
My old headers were warped and constantly coming loose since the exhaust pipes were wobbly. New headers and pipes made a big difference. I also used the copper gaskets. Fairly expensive, but I hope they were worth it. I tightened the bolts a couple times in the first two weeks and they seem to be holding fine. That reminds me, I should go out and do them again, just for fun...