Hi Midex.... Welcome to the forum. You have an interesting question as being from Alberta, we do not have a lot of laws regarding our vehicles....lol... My opinion (for what it's worth), and knowing only a tiny bit about US rules is: Although your 72 is an antique and probably grandfathered in regards to emission regulations, side pipes on a 72 were not a factory option so you are actually modifying the car. That probably puts you in a whole new category where they will likely want both cats and your pipes exiting behind the axles. If you require yearly inspections like some of our Cdn provinces, that may become problematic unless you know a friendly inspection garage. If you do, I'm betting that the local police have better things to do and probably wouldn't even know the 72 didn't have side pipes... If you get a ticketed, send it to me... I won't pay it bit I will frame it for my man cave....lol... jk... good luck.