An msd box will help a buch with the lower rpm portion of the test if you have any sort of cam in it.

The air screws on the carburetor make a big difference. Finding the sweet spot can be tricky if you have an engine that is borderline on passing. You can always take it to a service station and have them set it up for you with their equipment and then take it back and get it tested.

If all else fails buy some catalytic convertors without the air tube and splice them in as far forward as you can.

Testing old cars is a money grab if you ask me. Sure they pollute more than new ones, but the amount of time they spend on the road is very small. Most people drive them a few times a month in the summer to go to car shows and thats about it.