Here's a good illustration of the amount of error you'll be dealing with if you use a camera without enough optical zoom to measure frontal area:
Images resized so the width of the hood between the headlights is the same in both pics...
Left: 2x optical zoom
Right: 2x optical zoom shot through one side of a pair of binoculars from about 150 ft further away. (Sorry, don't know the power of the binocs).
If you can see the rear tires in the photo, it's a sign the zoom isn't strong enough. You can make out about 1 inch of the inside of the rear tire in the right pic, which isn't bad, considering the rear track is a total of 1 inch narrower than the fronts anyway.
If I could get a crisply focused version of the right pic, I think it'd be good enough to use the outline-and-pixel-counting method.