i bought a used catted downpipe from this guy about an hour drive from me yesterday. i was expecting him to drive up in a grand prix, or some random car like an explorer. turns out i was wrong, he pulled up in an 80's camaro that sounded BA. anyway the exchange was good and easy. i got home and didnt have enough time to put it in, so i saved it for today. i sprayed a little WD40 on the bolts, but it didnt help. i took off the motor mounts and pulled the engine forward, but the top bolts were still really hard to get at. luckily i didnt break any bolts. after i got all the bolts off, i tried getting the O2 sensor off. but i stripped it, so i just decided to unplug it and use the new one the guy gave me. i took the old one off, and put only the front 2 bolts and the hangers on. i started my car and almost crapped my pants cuz it was so loud and different. me and my friend drove aroud for like 5 mins but it was freaking loud. so we took it home and put the last 2 bolts on and thats it
TLDR: new downpipe is great, just a little difficulty with the bolts.