1974 was the year we started using catalytic converters. It has been the law ever since that happened that you could not modify the engine to affect emissions. And the issue is when you break the law who enforces the law? In my state Missouri the emission laws are checked by the vehicle inspectors when you buy your license. I was a former inspector, and we were just to do a visual inspection. If modifications were visible, then we were to flunk the car and they could not get their license. St. Louis and Kansas City areas had stiffer inspections where you had to pass a tail pipe test and then after 1996 OBD II you have to plug into your diagnostic connector and the inspection station checks for codes and a functioning system. Enforcement on older cars was pretty lax just because the inspectors did not want to mess with it much. I will give you one example I had a student who owned a S-10 he put a V-8 in it. He got pulled over by a state patrol and that patrolman was smart enough to know they never put a V-8 in an S-10, he impounded the car for emission violations. It is not that people have been breaking the law it is that nobody has spent the time and money to enforce the law. I never asked if the inspection station who passed the S-10 was ever penalized.