Hello from the US!
A few thoughts: First, there should be TWO catalytic converters on your car (one on each side). Did you replace both?
I want to make sure I understand - You cut the flanges off your original cat and welded them to the new cat?
There could be several problems - the most likely, in my experience, is that the angle of the flange is not exactly correct. Exhaust systems and components are built in a jig, in order to keep everything at the correct angle and alignment. If it is off by a VERY tiny amount, there will be a leak. Exhaust leaks are not always visible - a tiny gap can allow a lot of fumes to escape - possibly on the top of the piping, where you cannot see.
The 49-state catalytic converter assembly is different from the California assembly (because California does stupid things

). Here's the difference:
Right side, 49-state converter for your car:
Right side, CA converter for your car:
I thin the smartest thing to do would be to purchase the correct model for your application, OR use a Magnaflow universal converter (since you are not subject to emissions testing).
We'll help you figure it out.