In order to keep this car legal (smog) you may be required
to have the cat converter that matches the year engine
that you are putting in. If the car has a '91 manifold
then you would use '91 cat converter and exhaust parts.

From what I read C.A.R.B. is fussy about this. Other states
probably have similar things on their books for emission
equiptment. If a car in Seattle, WA fails emission test
after spending $100 in a repair center then comes back
for emissions and fails. an inspector craws under car to
check out exhaust and emission equiptment.

Gary