FWIW, my SMOG tech. and I do the following before any emissions testing. It requires some planning but not a lot.
1) run a full-tank of Chevron Supreme and a full-bottle of Techron;
2) 2-days before your test, change the oil & filter;
3) the day of the test, run your engine for a minimum of 20-25 minutes under load. If possible, do some wide-open accelerations up a long hill for 5-8 minutes;
4) if you SMOG test shop is NOT ready to test your car, ask them when they will be ready, because you want to be tested HOT (I will sometimes circle the block);
This has worked for my 1978 Dodge van, a friend's 1988 300E, and another friends 1989 Corvette. NONE were dyno tests, just a sniffer.
If you need to replace the CAT, I have been successful in using CATCO cats at about $80-$120 mail-order depending on application.