Just FYI though, the Pontiac GTO you have on the main shirt is a `73 not a `63. I can't really make out the other cars but the top one looks like about right, but the other 2 (I can't really make it out) look like the wrong descriptions. The bottom one I'm pretty sure isn't a Cadi. Its too small to really tell.
Why am I bringing this up? Because I think you have a legitimate market for these shirts. I'm in the autophile industry and I can tell you from experience that your shirts WILL sell at car shows and swap meets.
People are crazy about their collector cars and will buy anything that has a picture of it on there. Try `55-`57 Chevy, `65-`69 Camaro, `65-`73 GTO, any Mustang and any Corvette. Also, Plymouth GTX, Plymouth SuperBee, Plymouth Roadrunner, Ford Torino, and for a Cadi you can't go wrong with a `59. Want ideas for the kind of cars to do, go to a local car show or watch speedvision. RM Auctions will also give you ideas as will Barrett Jackson Auctions.
Hope this info is helpful. I look forward to seeing your designs on the circuit.