The OEM exhaust manifold bolts on the 6.2 have a triangle shape where the threads are...mine all had to have the heads sawed or torched off,they were rivets ,not hex any more..

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--then I had to pry and smash the old manifold with a sledge to get it to come off the remnants of the bolts,I also had to cut slots in the manifold where the bolts pass thru it...was hitting and prying so hard I feared I'd crack the head on the dang thing..it finally came loose,then it was a battle trying to thread the thing down and out from under the truck..it only fits one way,and just barely..
After I had the exhaust manifold off,I was unable to unscrew the headless bolts ,usually this method works great on a gas V8 and they can be unscrewed with your fingers once they are free of the manifold,or vise grips ,pipe wrench if they are stubborn..--no such luck with a 6.2..the vise grips just slipped,and so did the pipe wrench,and it wouldn't fit in to reach half the bolts either..
I had to braze nuts to the "studs" and the only wrench you can use on the bolts is a box end,because they are so close to the frame rail, a ratchet and socket cant fit..all the bolts did come out though,but hard every turn..took only 2 hours of agony leaning over the fender..my ribs ached for months after that miserable job..
The weird thing is the new bolts went in like I'd just tapped the holes clean,they were regular hardware store metric ones..10 mm.so at least they had some meat left on them and didn't snap..also the OEM bolts on the parts engine I got the manifold from came right out,and the manifolds looked nice..
I didn't want to use those bolts though,the heads were starting to whittle down on them..new ones were almost $15,--I hate metric bolts,they rape you for them here,and your lucky to find any the right pitch and length too..only one store 6 miles away had enough to do the job..