1) Engine in good mechanical condition.

2) Carbs in perfect condition, with no leaks past the throttle shafts (this is the #1 place they wear out).

3) Correct needles in the carbs for your engine, and carbs correctly tuned.

4) Valves adjusted accurately.

5) Timing adjusted correctly, and advance mechanism working.

I had a 122S in CA while smog standards still applied, and with all the above in place, it sailed through emissions tests easily -- not even close to the limits.

A catalytic converter will do you no good at all. In the mid-'70s, cars engines made to run very hot and very lean, which caused several oxides of nitrogen (NOx) as combustion byproducts. The cat reduces NOx emissions. An older Volvo in good tune doesn't produce NOx to begin with, so there's nothing for a cat to clean up.