I don't know ALL the reasons they won't fit, but lack of an O2 sensor bung would rate fairly high on that list.Will these actually fit a gmt400? I have a 1990 C1500, when I looked them up online it said they weren’t compatible
Didn't even have enough sense to cast-in bosses that could be drilled 'n' tapped; one for O2, another for a Vortec-style EGR fitting. Somebody was not thinking clearly.