Removing rusted driveshaft bolts

[Shorty]

I have just got the full rear axle from a sierra gt with rear disc brakes and bolt on driveshafts but having trouble removing the bolts from the driveshafts as very rusted.
Ive tried giving them a quick clean up to remove the thick of the rust but they are very tight and having trouble getting a good grip with the torx bit(40 or 45 I think it was)
Any ideas? heat or grind off are the first methods that come to mind but don't want to do any damage.