I wonder how many modified cars are also rustbuckets. In the NE, most cars will end up as rustbuckets thanks to the vast overuse of roadsalt every winter. Its not uncommon to see people hotrod rustbuckets without performing proper repairs or half assing repairs that quickly fail.
I took my '83 Monte off the road in '96. So, it spent 13 years and 12 winters on the road in the NE. When I checked the bottom out recently, I was surprised that after all of these years outside, and without a garage, the bottom of the car is in reasonable condition. The gas tank bolts came out with just a little penetrating oil. I even got a little bolt holding the ground wire for the fuel gage out no problem. That wouldn't have happened if it had stayed on the road here in the NE all of that time. I spent one winter down in Florida. The most dangerous stretch of road there was one leading to the orange plantations because the illegals were driving junkers along that highway to work. These cars had wheels that came off just driving to work!
With regard to mufflers, I would like to see one sound limit stated in decibels for ALL cars, trucks, motorcycles, lawnmowers, or whatever. A performance exhaust is the best way to increase performance. However, I don't want to get woken up at 4 AM by some A-hole either. There is no need to declare the exhaust system must be as quiet as stock either that way.
As for the cats, if the car isn't on the road much, so what if it doesn't have them? I consider maintaining old cars and keeping them on the road as saving Americana, parts of American History and culture. Keeping a stock emissions system on an old car is damn near impossible. Even if the knowledge of how it worked is maintained, it becomes impossible to get parts. It's not just the little do-dads, but even something as basic as the wiring harness that can't be found. If everything needs to be stock, then the manufacturer should be required to keep making those things indefinitely too. I suspect that there are fewer 75-86 cars on the road than 63-74. It is more money and work to keep them street legal, and less freedom to modify them. The result is fewer of them will survive for later generations to appreciate them. For those of us that consider the automobile a work of both engineering and art, that is a loss to us all.