He has the 5.7. The 993 is the Iridium plug we have talked about that supposedly does not fit the 7.4 Vortec but does fit the 5.7. Go figure!Compare the '993s to the old plugs. Is the threaded portion longer?
'993s are probably not the correct plugs. I bet the reach is too long.
'Course, you could have told us what engine the truck has...
Check with AC-Delco. Could be you're not supposed to re-gap the plugs. Problems with breaking-off the li'l Platinum nubs, or bending the thin center electrode.
It does have a longer reach, which I think you pointed out to me, than some of the other plugs for that engine.