exhaust bolt problem

[Snibble]

As I attempted to tackle my first summer project, I ran into a problem. The bolts/gasket/clamps that connect the cat to the center muffler are badly rusted and have essentially fused the two together. I figured I could take a hack saw and saw off the bolts, but from the angle of working underneath the car, it only let me do one of the four. Next thing that popped into my head is to cut the pipes behind the trouble area. I marked this with a red line in the picture. I briefly used a hacksaw and it did leave a mark, but not cut through. I was afraid that I might not get it to cut through with a hack saw. Can someone tell me the best solution to the problem I am having? How thick is the pipe at the end of the catalalytical converter?