If I was you I would send each of those links an email specifying your exact type of MR2 and engine it has.
At least then you have something if the parts do not fit correctly.
I know it's tight in the MR2 engine bay, but if you can take a few photos of the current catalytic converter, we may be better at helping you. I would like to see how it is mounted and where the sensor plugs in.
I would be quite sure Toyota did not install stainless steel exhaust parts on any MR2. It is possible that whoever owned your MR2 before you changed it though. This happened to me when I bought my MR2 and needed a mid-section. The Toyota part did not fit to the aftermarket parts.
