Use a rose bud tip on a oxy/acetylene torch. Heat it up cherry red and start wiggling it around. You will have to heat it several times, but you will get it expanded back out to get it off. Concentrate where the u-bolt has made a crease in the pipe, you need to get it back out to get it to slide off. I worked at mine for about an hour to get the cat, muffler, and tailpipe apart.