So, I feel I should give a more in depth explanation of the install. Mine is an 09 so there may be some differences from older ones.
I started at the rear of the Y pipe, there's a rounded band over a ball type connection in front of the flex pipe, loosened that and separated. The clamp is help in place on top so don't try to remove it. Then disconnected the O2 sensors, then separated the joints at the shorty heaters. Found out the hard way that the Y pipe doesn't come out so I supported the transfer case and removed the transmission mount nuts, the the 2 right side crossmember bolts then the inside left crossmember bolt and loosened the outer. Small pull and it dropped, pulled out the pipe and re-installed crossmember.
Removed the plugs and wires and removed shorty headers.
Slid left side header up from the bottom and attached with new Mr. Gasket header bolts. Reinstalled plugs and wires. Same for right side.
these headers have the flange welded to them on the end with a short pipe extending past the collector flange, slid the doughnut gasket onto this. The left side header is a few inches shorter than the right, dunno why, just is. The cat, or resonator if thats your choice, is welded with a 90* bend after. There are 3 holes for the O2 sensors, one before the cat and 2 after. I installed the front sensor and put a plug in the first aft sensor. The first aft sensor is right next to the cat, the second is about 6 inches behind that and that is where I installed my O2 sensor. I put the cat up to the pipe and finger tightened the bolts on the loose flange so it would stay in place but I could move it to line up the slip fit cross pipe.
The right side cat is only the cat welded to the bell fitting with a loose flange, it only has the 2 holes for the O2 sensors (the 3rd is in the Y pipe). Same here, finger tight for fitment with front sensor installed and first aft hole plugged. The end of the slip fit on the cat is just before the crossmember.
Now for the slip fit of the Y. Slid it above the crossmember into the rear of the right side cat, rotated it so the cross piece going to the left side was below the slip fit for the left cat, lined em up and slid it in. Did a little pushing and pulling and got everything where I wanted it. The cross pipe was hitting the right side aluminum transmission gaurd just a bit so that got bent a touch and out of the way. Once I got it situated where I wanted it and was happy, I tightened the band clamps on the 2 slip fit joints.
The company put in a flanged adapter at the rear of the Y that is at a slight angle to the ball joint on the flex pipe. Finger tightened the 2 flange bolts, aligned and tightened the ball joint and tightened the flange bolts. This is when I installed the rear O2 on the right side. Plugged in all the O2's but the right rear. Because I used the rearmost ports, I had to open the wire holder behind the wheel well and slide the corrugated wire covering down a few inches so the plugs would reach.
Fired her up, checked for leaks, let her idle down for 10 minutes then took her out.
Hope this helps.
I'll do my best to answer any questions about it. Wish I had pics to help but like I said, all on my phone and site won't let me upload em.