Found on my son's car that the two-piece triangular clamp that attaches the cat to the front part of the exhaust had developed a leak. In fact, two of the three bolts had rotted away. The two pieces are inside a flange on the exhaust, so you cannot get them off. I was unable to get the bolts out and started drilling them out (two are done, one left). Is there an easier way? Is there a some type of replacement part or is drilling the only way to fix?

Thanks.

You can get a replacement flange for this. The car parts store has a triangular flange in three pieces that assembles on the pipe. As you mention you can not get a one piece one over the flange. Get two of these at the parts store along with a new donut gasket to go in between the flanges on the pipe ends and you are in business.

The original arrangement for the cat to front pipe connection is not actually a clamp, but two flanges that mate together with a gasket in between. They are held together by three bolts, with the flange on the cat (I think) being the one with the threaded holes that accept the bolts. Aftermarket cats have unthreaded holes so you need to get nuts and lock washers for the bolts.

If the bolts are rusted away the best thing would be to drill them out and replace them (of course, get a new gasket in there because that's probably what's causing the leak). For bolts, lockwashers and nuts you'll want to get the high-temp gold-ish colored hardware (sometimes called type 6 or schedule 6 hardware).

If by "front part of the exhaust" you mean BEHIND the cat (cat to front muffler connection), then I think that connection is held together by a common exhaust clamp, which you can easily just saw off with a hacksaw and get replaced for a buck or two at any auto parts store. Bring the old clamp in because they come in various sizes.