alan,
I don't remember the movie well enough (I thought the Valiant was a Vega, for example, duh...), but I believe it if it was filmed at under 40 MPH speeds. I remember that in a lot of older and old movies and old TV shows, "Killdozer" for example, you could see that the camera speed was turned down a few frames per second when action scenes were filmed to make the action look faster.
As for Valiant vs Vega, I'd guess there are a lot more Valiants around today than there are Vegas. The sleeve-less aluminum block was too far ahead of materials and manufacturing of that time. I have a some bad memories of Vega engine replacements. Thanks for remembering the Cosworth Vega, I've read a few funny stories about how that ever happened. A V8 Vega would have to be tube-framed or front clipped to make that fit.
I think I can 'one up' you on the neighborhood guy with three Vegas. A few years ago I bought an old car from a guy, and had to go to his home to pick it up. I asked for directions to get to his house, and he told me the sort-of 'turn left where the old ice house used to be' kind of directions. He finished with "...You can't miss it, there is a Pacer parked in the driveway." He had three Pacers. He proudly pointed out that they were all LX's, that he didn't own any 4 door Pacers!
(everyone else, sorry for the thread-jack; Vegas and Pacers on the Dozer forum. Ugh!)