You can make old cars appear on any road spline that supports traffic by putting the kuid numbers for the old cars in the map config file. I can't remember exactly how but I am sure someone will let you know.
I am prepared to be corrected here but I do not believe you can restrict a road spline to showing a single subset of cars. You can add additional car kuids to the map config.txt file, as suggested, but these extra cars will only be added to the default set that would have appeared anyway.
Well I had better correct you then. If you download drivable vehicles from the DLS you can replace the kuids of the original cars with the kuids of your downloaded cars, therefore none of the original cars will appear if you replace them all.
Thanks for the tips. But....I remember a software program that will convert all road splines with traffic from modern vehicles to old vehicles. Once the progrma is loaded all road splinew will have old cars. In order to convert back you have to download the original spline for DLS.
Gary