I used the u-clamps and cement and put them on good and tight, which I regret now as I can't get anything apart!
The flex pipe is not the bendable stuff used for temporary repairs, it's a proper steel braided line with the corrugated interior, so I figure it should kill the noise as well as the original. I think I'll take it to the welder tomorrow and get him to seal everything up to see if it gets better. The sad part is it's getting close to the cheapest quote I got from the muffler shop, with tons of wasted effort on top. At least I managed to save the cat and oxygen sensor, so that's several hundred bucks right there if I can make it work for her.