Hi Jay,
When a Cat and/or Muffler plug it causes a tremendous amount of back
pressure.  The engine over heats and then the oil thins out, thus a drop in
oil pressure.  It will cook the engine!  Been there, done that.  Over a
period of time soot/crud builds up on the leading edges of the imput side of
the ceramic honey comb openings inside the Cat.  This will restrict the
exhaust flow and lead to an increase in back pressure. It is a good idea to
inspect or replace the Cat when putting in a new engine and could even
restore some power in a high mileage engine.  A new Cat can be had for $100
and is cheap insurance.
-Kevin Dawson