Here's a link for you: http://www.mrpinhole.com/calcpinh.php
With this, your exposures will be a little less guessing and a little more knowing. Photographic Paper can be assumed to have a sensitivity of about 5ASA.

Btw, I love your camera... everybody can take photos with an old shoe box, but using a little truck with the license plate as shutter? That's bordering on genius

Poking a hole directly into the metal will always give you such soft pictures. The smaller the hole, the sharper the picture (until a certain limit, see "optimal pinhole diameter") and your's was way too big and the edges are probably to uneven. The calculator can help finding out, what's best for your needs.
Using a small piece of aluminium foil or other very thin metal sheet is the usual practice for making small pinholes. Most people don't push the needle right through but use it to make a tiny dent which is then filed off. That way, you get very small and even holes. I usually just take a sewing needle, place it onto the aluminium foil with very little pressure and turn it around while placing my index finger on top (not pressing hard, just adding a little weight) and repeat that from the other side to smooth the rim... i can usually get almost perfectly round holes from 0,15 to 0,6mm in diameter that way.

As for your problem with the half-overexposed picture: It's either uneven developement (ruled out by multiple tests, I guess) or a light leak on the top of the camera, close to the film plane. Try putting a small flashlight into the box and taking it into a dark room. Does any light get out, apart from the pinhole? Then you've got a light leak that should be fixed with duct tape, a few drops of black paint or whatever you happen to have around.