What are the rules in the U.S. if you drive a Car that was made before Cataylic Converters?

I always think of the Bluesmobile the 1974 Dodge Monaco Police Car with the 440ci Magnum

Remember what elwood said about it "It's a model made before catalytic converters so it'll run good on regular gas."

I have a Friend who drives a 1974 Oldsmobile Delta 88 455ci Rocket also a Vehicle made before Catalyic Converters.

As i read they were mandated on all Vehicles made in 1975.

So what if you are driving a vehicle such as a '74 Delta 88 in any state let's say. That does not have a Cat are you breaking the Law???

Now what if you are Driving that '74 Delta 88 in California whether you are a resident or just visiting are you breaking the law in either of those situations???

Can you be forced to put a Catalyic Converter on a Vehicle by the Government in any state???

Just curious how does the emissions check work in let's say California do you have to get your vehicle tested and get like a signed document that you have to take to the MVD in Cali for them to allow you to register your vehicle or what??? how does the process work???

I heard from somebody that if your vehicle is pre 1975 no one not even the government or even any of the Strict California emission laws can make you put a Cat on your pre 1975 Vehicle is that true??? Someone told me this and they said the reason was is that because the vehicle at the time of production no matter if it was from the 1910's'1920's,1930's,1940's,1950's,1960's,1970's they were made and made to follow emissions rules at the time of production and it's only suppossed to comply with emissions rules at the time such as 1974 emissions and not be changed to present day emissions the vehicle is just engineered to operate they way it was with those emissions standards. I was also told that's why Vehicles have those stickers on the door jams.

Sorry for that confusion it just means in a more simpler way i was told that a 1974 Vehicle is suppossed to comply only with 1974 Emissions PERIOD even if you are driving it in 2013. is what i was told

There i hope that simplified things sorry for all the ramble.

I just want to hear from some peeps here.