[Jim Sims]

I don't believe you need heat and if used it may end up damaging adjacent rubber in the boot or in the transmission output flange lip seal. Ensure the hex wrenching features in the bolt heads are completely clean of debris so the hex bit will fully and deeply seat. Use a hex bit in good condition with new sharp hex corners; grind the bit down if necessary to get to non-rounded corners. Apply inward pressure to keep the bit seated in the bolt head while turning - sometimes it is helpful to have a second person; one keeping the bit seated and the other pulling on the breaker bar handle. Use a breaker bar with a longer handle (~18") for more leverage; usually 3/8" breaker bars have handles that are too short for this task. If the screws were overtightened they should be replaced as they are likely stretched. If the wrenching feature in a bolt "rounds out" one can sometimes gain purchase by tapping in a torx or triple square bit but this usually ruins the bit used. There is also room to use a good pair of visegrips firmly clamped on the outside of the head of the bolt to remove it.