99 626 ES 4cyl cat converter problems

Hi all, I was wondering- I got a 421 error code when I did a read on the ecm which means I need the 'upper' cat converter replaced (this is the first time I've heard of a car having two) - I priced it at the dealership, $416.00. Then I called Autozone and they have a universal fit that supposedly fits both spots (upper- after manifold and lower- under body). Has anyone had any experience with this and if so, please let me know if this is feasible. I really could use the savings. Thanks in advance! J