There are probably others here that can give you much more detailed information than I can but I'll start out by telling you that 1/320 is way to high of a shutter speed to get a pan shot. You should be somewhere around 1/60 - 1/50th of a sec to get good background motion blur.
As far as the auto focus goes. I think your better off with manual focus than you are Auto focus. You said your camera doesn't have Auto focus... did you mean it doesn't have Manual Focus?
When you pan with Manual Focus you can pick the specific place you want to take your pan shot, focus the camera there, then when the car comes by and your panning you know your focus is set correctly.
Make sense?
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