I am not sure about the whole in the cat, I had to buy a California cat so it would pass smog and it was just like my old one.

But as for the pipes, I had a lot of problems getting them to fit right. My only suggestion is that you push all of the pipes together before installing anything on the car. I thought I had everything pushed together enough but after bolting things up I found that I hadn't pushed the 2 pipes that go into the muffler enough and the system was too wide to fit in the proper place. So, make sure you measure the total width and see if it will fit before installing.

The O2 sensor is either in the downpipe about 18" away from the exhaust manifold, or in the cat (or immediately upstream of the cat). If your O2 sensor is in the downpipe, then just get a properly-sized plug to fill the hole in the cat. Stainless steel is probably indicated due to the temps.

When I installed the IPD sport exhaust on my 245, I found that the axle pipe did not have adequate clearance with the axle. I lowered the muffler hangers with some bar stock extensions. I removed the existing hangers, and got some metric holts of the same length plus a pair of nuts with the same thread. My extension was 1/4" bar stock (what I had on hand; 1/8" might do) x 1", and about 2.5" long. Each hanger had 2 holes drilled in it; use the new metric bolts to attach the hanger to where the old hanger had been, then bolt the original hanger to the lower hole using the new nuts. I think I lowered mine about an inch and a half, and never had a problem with it since.