Get a length of pipe just longer than the cat and that will just slip over the exhaust pipe. Use u-bolts to clamp the straight pipe in place. You're doing this because you won't be driving on public roads right?

Or cut the pipe close to the cat so you can rod it easily then put it back together with something like this - http://www.summitracing.com/parts/WLK-33240/

Sixto
87 300D