Figured I'd make a thread with some thoughts, as perusing the old ones helped me out along the way, so if it helps one person then I done good. Pretty much did this all by myself with the car on 4 rhino ramps.
I bought the 1-7/8 ARH's with the catted X through RDP store, fairly smooth transaction and at a really good price. I mean, 10 years ago I bought the same setup with a catted y for my Fbody, and this setup with quite a bit more piping and inflation was only about $200 more. So props to those guys.
So to start, this is my daily driver year round and a NY (aka salt belt) car and has been through 3 winters. The hardware on the pair of clamps at the rear mufflers was completely rusted to the point the whatever the strange fitting size of that Euro spec bolt was not going to be turned by any socket known to mankind (on the new ones a 12 point 10mm works). So that scrapped my install plan before it started. Ordered a new pair of the Mercedes clamps for ~$15/ea as noted in one of the threads on here (as the GM replacements are $150ea, no that's not a typo). I had to use a cutoff wheel on each to get the old clamps off. Also in prep for this I bought new front O2's as I suspected they'd be seized in there pretty good, and that was the case. So my recommendation before you begin is take a gander at your muffler clamps and see how bad they look. The bolts on the studs at the manifold end weren't in great shape either, but I let them sit overnight in PB Blaster and after a tense will it snap/strip moment each ended up breaking loose. With that done getting out the factory midpipe was simple - btw nice quality stainless GM, looked like it would've rusted through in another 3-4 winters.
Driver's side manifold was fairly simple to get out. Passenger's side had the known O2 connector challenge. So here's what I did - cut the O2 wires to get the manifold out first (another benefit to just buying new O2's). Now, I could see the benefit f you can wrangle a wrench in there just unscrewing the O2 and reusing it without disconnecting. Probably wasn't happening with mine. Being blessed with small hands I could find the connector blindly by locating the "christmas tree" clip. Knowing from other threads that the connector was clipped to the head I slowly pulled and worked it loose until it popped off - nervous that I destroyed a wiring harness and the car would never start again, but I yanked the right one. From there was easy to plug in the O2 extension and feed it back down.
Driver's side header required unbolting the steering shaft - order a new bolt ahead of time and torque it to the 37lb*ft. Experience on the TA tells me not getting this right will give you sloppy steering feel and annoy the heck out of you. Use bungee cords to keep the steering wheel in place. Use another set of bungee cords to keep the shaft as far up as it will go. A jackstand on the header in the absence of another pair of hands will help get the header close from the bottom, and then from the top pull the header up and move the steering rack down. Passenger's side was straight forward. After fighting the **** dipstick for a half an hour plus and then getting a spare set of hands I pulled up an old thread and noted it DOES NOT go thru the motor mount. Within 5 minutes it was back in.
Another suggestion - rent (or buy) a 3"+ pipe expander ahead of time. ARH has good fitament but at least 3 of the connections were very tight including the driver's side collector. The driver's side catted pipe wasn't going to go on without opening up the pipe a bit. Same with the X connections. Possibly one midpipe.
Push the catted pipes to the X pipe first and tighten the clamps enough to hold it together but loose enough to allow some movement if pressure alone isn't enough. Getting this on each collector and in the hangers all at once is probably the toughest part of piecing the exhaust together IMO. As mentioned in another thread, at the slip connections make sure you can see the inner pipe is fully past the cuts on the outer pipe. Once you do that use some wood shims to space it from the floorpan near the cats. Don't fully tighten any clamps yet. Now get a pair of jack stands and put the midpipes in the ball and socket connection. Then connect the last pair of pipes to the mufflers at those ball and socket connections and clamp it tight enough to hold the pipes together but loose enough to allow movement. Now go to the midpipe ball and socket and tighten them enough to hold in place. At this point I now moved the jackstands to the back under each muffler and used them to hold the mufflers in place as desired. Then went back and tightened the 4 ball and socket clamps fully. At this point now I started at the primaries and fully tightened each clamp. Removed the jackstands and shims, and everything was good and solid in place. Went in and zip tied the O2 wires, as well as one of the starter wires wrapped in reflective tape that was too close to the primary for comfort. Also removed the plastic block on the driver's side inner fender well under the hood and zip tied those out of the way. Shh don't tell anyone but right now the chassis brace is out
. The spacer's still weren't enough for clearance, will have to look into an aftermarket one.
Installed the updated LMS tune, fired it up and smoked up the joint nicely from burn in. Through open mufflers outside at cold start its a hair raspier as expected but volume wise, I was pleasantly happy that it really wasn't any louder at idle or cruise/highway rpms below 2500 at least inside the cabin. 3500 and up though it really starts to scream. My SOTP meter has never been great but hammering on it in 2nd (A6) breaks them loose very easy now so it has gained noticeable power. No rattles or exhaust leaks, but will retorque header bolts and clamps as needed after a few heat cycles. All in all though no complaints and quite happy.
Think that's about everything, any questions ask away!