The bolts are 9/16", which is actually 14mm. My recommendations would be to pull the heads with the exhaust manifolds in place, and use rust remover and penetrating oil on the bolts. With the bolt heads clean, you can get a good 6 point impact socket on them and impact them off. Me, I'd let them soak for awhile, and maybe use some heat from a propane torch, too. Again, you may simply have a burnt exhaust valve. Not a big deal to fix.
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Jeff 
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