I have a 98 f150 4.2 V6 2wd auto and have detected an exhaust leak that sounds as though it's coming from the passenger side cat. There are 4 cats total on this truck, a smaller "pre-cat" and a cat on each side.

I was planning on getting flowmaster 40 and new piping (sticking w/ single exit pipe) due to a shoddy job done by previous owner on exhaust, and figured might as well replace the converter while i'm welding...

If I replace the converter(s), will it make a difference with the O2 sensors or engine performance if I only put one aftermarket converter in on each side instead of a pre-cat and cat?

Also, are there any specific brands or models that may be more suited to this application. The stock cats I think are oval shaped... all 4.