if you can get the bolts out of the cat join you wont have a problem. I had to drill mine out, you could use a nut and bolt if you bugger the threads on the cat but i only drilled half way and it came loose.
But worth checking the clamp by middle boxs see if the bolts will undo, if not get new ones. Only a couple of quid.
Also important to have good jacking facilitys. You need the car high enough to get your chest under the sil. On stands obviously.
Ps. Just remembered. I only snapped one bolt. The first one i undid. I then was more carefull with the other 3 and got them out by undoing until the thread went tight, and stoping, then back in half as much until the thread went loose, then undo until thread tightened and then tighten in until it frees off and so on. Like tapping a thread but in reverse. It just gives the threads a chance to clear the rust and crap before you undo again. Obviously a good clean of the threads with a wire brush and some plus gas or penetrating oil will help, if the nut has not rusted away completely of course.
Hth