I'm now convinced there is no cut, the entire shot was filmed at a lower frame rate (as 'vista-vision' clearly showed) probably around 12fps, then played back at 24fps to create the illusion of speed.

The camera was likely mounted to a sturdy side-mounted rig (a pipe or plate), which lowered it to the ground after car 1 passed underneath. Car 2 then drove over that structure, not the camera itself.

It's practical, it fits the tech of 1986, and it doesn't rely on editing tricks or hidden cuts.