At my shop we bend pipe for exhaust systems. Bigger pipe is not necessarily better. The right sized pipe will draw the exhaust out of an engine, too big a pipe will allow it to slow down and cause back pressure. I run a Hedman header with 2" pipe back to a NAPA Quietflow muffler with a tail pipe. I did weld in a cat, even though we don't have emissions testing here yet. One county away, but not here. Of course, the cat is only $30 my cost. But here is one thing to consider: Having the cat mounted close to the collector keeps your O2 sensor hot during extended idle and off idle running, which means you stay in closed loop.