Catalyic Converter Question

ok i got a question here...i was going to put a new converter on my impala yesterday and i crawled underneath it and i was looking at the Y pipe that comes off the converter and then connects to the resonator can thing.....now...i see the oxygen sensor after the converter is going to be a problem....with a direct fit after market cat, will it have a place for that oxygen sensor...i can't just simply cut out the old cat and clamp a new one in because of the Y pipe. It looks like it's cast iron but i couldn't get a really good look at it because the car sits so low to the ground. Also the Y pipe looks like it has an oval shape to it and i don't know if i can simply clamp the new cat to that.....what have you guys done to get around this...

let me know,
mike

What year Impala?

It's a 2000 base, 3.4 liter impala.....let me know

mike