I had to use them on my 87 GMC Sierra Classic TBI 350.
I also had to raise the throttle cable bracket so it would idle down correctly. It was idling at 550 before, and 1200 rpm's after. I had to buy a longer bolt and 6 flat washers between the intake and the bottom of the bracket. When I got it back to the original idle speed I also had to reset the TV cable so it would shift correctly.
I recently replaced the head gaskets and took the air raid off. With as many miles as I have on mine (387,000) I couldn't really tell it was doing anything for the performance.
My Brother has an 88 GMC Jimmy 4x4, everything looks the same under the hood as mine, and he didn't have any of the problems I had.