The main way to change this is to strip the "required" parts and have that as truck2.pod on it's own and just remove the trucks.
The problem with doing that is you run into a big risk with many many custom trucks. A lot of them do reuse elements from truck2.pod in addition to the mandatory ones, as was discovered with the JTruckViewer when it was randomly not rendering a truck correctly. So removing the stocks from truck2 is finnicky. It CAN be done but one has to go bit by bit to see what is safe to be removed and what is not. The bodies themselves are usually safe but you have to keep the tires for sure as many addon trucks make use of those tires. And with the amount of stuff to do, this is definitely not a priority
It should be noted that the game also looks for the actual .TRK files for the stock trucks when loading tracks. If you notice, truck starting locations are associated with specific .TRKs in the .SIT files. Those trucks need to be podded or it will crash. This was the issue I was having with my 2.1 trucks, so I had to backup all of the stock .TRKs then rename my trucks to the stock names (i.e. "BIGFSS.TRK" to "BIGFOOT.TRK"). This also meant that I had to use the WCW truck .TRKs for the red Snake Bite, Predator, etc.
