i think it's always worth t looking for some blueprints of the car, a site i recently discovered
The-Blueprints.com - 26351 blueprints online has a lot of usefull stuff they often include a frontal view
sometimes even with measures but if not these can be found and applied the the drawing
if you're working with pictures and you know the with and height of the car, you could set it to the correct width in photoshop and that stretch/squash the height untill this matches the scale of the width... this still leaves room for errors but i imagine if you match a crappy 3 view drawing with a picture and real measures and than do a pixel count your arrive pretty close to the real value