Heat the box with a torch. Soak in Kroil. hit the top of the bolt with a hammer to help the Kroil get into the threads. Next day, carefully turn the bolt clockwise, feeling for the slightest bit of movement, then stop! Then try to back it out. I don't know why tightening a bolt breaks it free better than loosening, but it does.
If you don't get any movement when you carefully tighten it. Start from the beginning and do it all over.
52Carl is right on, except he missed on very important tool. PATIENCE!
In this case an abundance of patience is required and of course a 6 point socket. Don’t attempt it with a 12 point as you will round it out. Heat the area surrounding the bolt threads to two - three hundred degrees then douse it in your favorite penetrant. I like Kroil, followed by PB Blaster. Now tap the head of the bolt. Try to remove but don’t get too forceful. More penetrant. Let it cool, try to remove but again, don’t get to forceful. Douse it in penetrant and let it sit a day. Repeat!
If after three or four days of doing this, cut the head of the bolt so you can separate the two manifolds. This will leave you with the end of the stud sticking out. Find a nut that just fits over the stud and weld it on. The heat of welding usually breaks the rust bond. Douse and repeat the above procedure.
Good luck!
