You have two choices here..
1.Get out the drill and start using foul language because it's taking forever
2. Get some heat on the subject, and Oxy-Acetylene setup would be best if you have access to one. Heat the area around the seized bolts on the flange until it glows red (be careful not to melt it) then give it a smack with a hammer and it will shoot out with no problems.
Good Luck
