[V8] More about Cats, mixture, etc.

[m35a1@juno.com]

I'd been following the thread about the ruff idle, rich mixture and poor
response as my '90 has been doing the same thing.
It did it when I first brought it home; changed the O2 sensor and all was
well. For a while.
It started doing it again recently and I thought I'd look into the cats
as it felt as though there was a blockage in the exhaust system.
I removed the cats and core-drilled them; they are now nice and open. I
blew out the muffler/tailpipes in both directions (hi-volume air, i.e.:
the ShopVac), also nice and clear. Changed the O2 sensor again. 
No improvement. Matter of fact, now I have a nice exhaust leak at the
coupling of the tailpiece of the cats into the system.