middle factory manifold bolts

I am removing a factory exhaust mani and the two top middle bolts that are difficult to get to, started stripping, despite my best effort to be careful. anyone have a trick to getting them loose? i put heat on them, shaved a 12mm wrench thinner for more clearance, gave a few taps with a rubber mallet. Im not sure what to do now.
 
Once you have all the other 6 bolts out try tapping straight down on the manifold with a dead blow hammer. Those two bolt holes are slotted out the top. I have had that happen before and with all the other bolts out the gasket gave way and let the manifold slip out. Use caution and common sense as to not break the cylinder casting or manifold. If that’s a no go, use a carbide burr to hog the bolt heads off and remove the manifold. Then a vice grip on the remaining bolt section to remove it.
 
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