I'M with Mr.Z on this one;
i'm a chevy person first and foremost, but i am, and always have been obsessed with anything to do with GM. Especially pontiac, oldsmobile and, of course, GMC. So keep the questions coming.
i'm 98(no pun intended) percent sure that the 98 and 88 used the same frame except the 98's frame was a few inches longer. Actually, if you look at the front ends of the cars, they are probably identical---as with the doors and roof. Only the tailights, rear fenders and trunk are different. Most of the parts on both cars are interchangeable.
So, i'm willing to bet money that the headers will fit.