Good news! I only had to cut a notch in one of the bolts, the other 7 came off with a big Phillips head socket wrench. But they'll still get replaced (+ anti seize).
But I'm a bit puzzled, because the o-rings look fine and probably recently replaced. There was some dirt caked between the intake and head, but outside of the o-rings. Could be a clue that they weren't sealed tight enough even though they're in good condition?