Factory GM exhaust manifold bolts have a slightly different shoulder and should be installed with thread locker and torqued per the manufacturer's recommendations. Not knowing any better, I always try to stay with the factory fasteners, when it comes to engine bolts.
For quick reference, three identifying marks = Grade 5, five identifying marks = Grade 8, per Carroll Smith's book entitled Nuts, Bolts, Fasteners and Plumbing Handbook. I have found this book very informative with regards to everything you need to know about fasteners.
I hope this information is helpful.
Tony Smith
4-5-6 Chevy Trucks
