Welcome to the Wondefull World of Task Force Body Panel Alignment!
The doors on the '55-'59s are a real pita to get straight, and it doesn't help that the factory doesn't really give decent alignemnt adjustments to get it right. As noted, get a helper to assist you. One thing that I found to be a real problem with adjusting them is that the door has to be open to access the bolts, so you really can't tell where you are really at. I actually gave up on the factory adjustments, and used shims behind the door hings to get it close. I know it isn't right, but it did work.
BTW, if you ever see a TF truck with perfectly aligned doors, you can bet that someone spent alot of time fitting them, in both hinge adjustments and also reshaping the door to fit the opening better.
