Well, we know that movie-making is all trickery, so when we cut to a shot on board the truck, it's reversed its direction and is actually travelling south on Magnolia instead of north. Undoubtedly, the lighting dictated this choice. By still-framing this sequence you can see some of the familiar downtown Larkspur stores whizzing by. Here, we've just passed the phone company at 464 Magnolia at the left, and now the Larkspur Hardware has come into view. Earlier, you can make out the Bank of America at the corner of Ward St., Agner's Variety and the Larkspur Pharmacy.
The guy to the right of Brian Donlevy is either a real-life Larkspur fireman or a stuntman; in either case, a close examination of this sequence shows that his main job is to hang on to Donlevy so he doesn't fall off the back of the speeding truck.
At any rate, if you can identify him, please email me by clicking here.