1990 xj40 dual exhausts?

I've been under the car a few times this past week, patching a small
exhaust leak and doing other repairs, and I'm sort of wondering about
the design of the exhaust system.

One would think from the two pipes out the back that this car has a
"dual" exhaust system.  It does have a split manifold - three
cylinders each into two manifold exits, but then I guess it y's into a
single pipe because from below there is a single pipe with the oxygen
sensor mounted on top.

Then it splits into two again with a real noisy - read power-losing -
y-pipe with a couple of larger chambers under the passenger
compartment, then pipes past the rear axle to larger mufflers just in
front of the rear bumper.  This y-connection was a tad noisier than
stock, as I have just cemented up a small hole right where a shroud
mounting bracket was welded on to the pipe just behind the y.  If I
ever take the exhaust right off I'll weld it up, but for now the
"bubblegum" repair works just fine.

There are two layers of shrouding on my vehicle, both laid on top of
the middle section.  I suppose whoever worked on this car in the past
couldn't, or didn't for some reason, bolt these together one above and
one below the middle section.

Those larger chambers in the middle - are they the catalytic
converters, or is that the single pipe with the o2 sensor on top, and
the offset chambers are merely part of the noise suppression
apparatus?

I'm wondering at some future point, when this vehicle is old enough to
be acceptable where I live as a "classic" aged vehicle, and no longer
requires an emission test, if I can re-plumb it as a proper twin
exhaust vehicle again, or will this just be too hard to do?  Are there
parts available for this?  Would I have room for 2 pipes at the
section where the o2 sensor is mounted?  Will I have to make up some
pipes of my own?

Another thing - some where I read (the manual?) that this car has a
stainless exhaust system, but the middle section is rusty old plain
steel.  Is this a replacement that is not up to original factory spec,
or is it only the rear section of the exhaust system that is
stainless?

Comments welcome.

- Tom