I rounded the one on the head side when I did my EGR.
Sadly it was the least accessible of the two allen bolts that fit into the exhaust manifold that turned out to have a head made of cheese…
The allen bit just turned and rounded it out, so I tapped in a torx bit, and that just rounded it out that bit more. So I tapped in the next size and of course that rounded too.
I did think about cutting the pipe to improve access and see if I could get a pair of grips on it - but then I'd be committed to getting it done and I needed the truck for work the next day.
So I used a hack saw blade and started to cut a slot into the head - I had about 5-10mm of movement - so I did that until it was becoming mental torture and packed in for the night. I figured I'd drive it up to my friends, cut the pipe off, and try to weld a bolt onto the head.
Next morning I thought I'd have another go with the hack saw blade and realised I'd made half decent progress the night before. So I tapped in a screwdriver bit and turned the ratchet … and it started to round off the head.
Oops - running out of options. So I knocked in the largest scredriver bit I had - started to turn - it started to round … so I reveresed it and tightened it up a bit and then BINGO - it came un-done.