[75 911s]

The transaxle flange is steel. Try to hit the backside of the thread with penetrating oil in between the flange and the trans.

Smack the head of each bolt straight on (lightly) with a flat hammer to break the rust loose.

Heat the flange a bit to make it bigger and then use an air impact. Make sure you are fully seated with the bit as you said. Wurth makes a cooling product that you can make the bolt smaller, or you could use carb cleaner. Not sure if that might make it brittle though.

Can also use a long breaker bar. That's what I usually use. Make sure your ebrake is on.