Replaced a catalytic converter on my 2000 Chrysler 300m.

New to this forum and new to replacing catayltic converters. The joint between the CAT and the exhaust was leaking causing a loud noise. I searched on these forums and it appears this is common at this joint. The user said he welded the joint. Anyway, I was unaware of the possible welding fix so I went ahead and ordered a Magnaflow catalytic converter assembly from Amazon of all places, this one:

http://www.amazon.com/gp/product/B00...00_i00_details .

Seemed like a piece of cake. Had a little trouble installing and ended up replacing the o2 sensor since I was unable to get the old one loose. Got everything put back together and took it for a drive. Ran fine until a half mile down the road, it started making the "leaky muffler sound" and lost power. I figured the marmon clamp didn't hold. I managed to make it back home. It has an odd smell now and makes a rumbling noise. Everything seems to be tight. Does this need time to break in or did I purchase a non-compatible CAT? At a loss right now and will more than likely be taking it in for service, as I should have done in the first place. Thanks for reading...