Sadly, the two cars I'm missing won't show up where it's mentioned in this guide.
Cadillac Lasalle Series 50 (4 Door #1)
Mercury Custom (Sport #3)
I can't find them at all, throughout the whole game I've never seen that Cadillac with the little bumps on either side of the front.
It doesn't help that since this is based in the 40's there isn't THAT much of a difference from car to car.