My exhaust is a little noisy despite all the new exhaust parts. (Head pipe > on back, plus cat and 02 sensor)

Is it possible that I installed the cat wrong? For some reason I'm not totally sure about the way I put it in, the 02 sensor points in a different way than when I took it off. A Car X-like place put in the old one in, so I'm not sure. They have a tendency to put the wrong part in. Is it possible I put it on the wrong way? Would the incorrect "flow" make it a little noisy?

Now, I have to find where that leak/leaks are coming from. Yes, I'm using the included clamps. I've heard that clamps suck. But, I sorta don't want to weld everything in case I have to replace certain parts down the road. Plus, a don't have a welder. My friend does, however. So, what is good to weld, and what should not be welded?

It seems there is some gurgle coming from the place where the under suspension pipe meets the rear silencer. I've seen a little carbonization and water coming from that seam. I've adjusted, re-clamped and done all that I could right there. But, I've also heard that the system will "rust" itself silent. What's the good word on all this? Or did I ask too may questions?
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89' 245 DL B230 5 spd., brick-red brick