Seems that they need grinding regardless of whether you use them on a Chevy or GMC, especially with an Offy intake.
I'll pass along 3 things I did that helped on mine, 1- replace the 2 outside guide pins with bolts/threaded studs. This gives a little more even clamping and will help stop leaks at the outer ends of the manifolds. 2-Replace the other manifold bolts with studs. Makes for easier installation and seem to torque down better that the bolts do. 3-Forget using the original manifold clamps, they're just a pain to make work right without exsessive grinding, followed by extensive foul language when they don't fit right. I made new ones from 1/4" stainless flat stock. Much better for clamping the manifolds and they won't get on a funny angle or try to shift on you when you tighten them down.
