Some background information: wescottsauto.com > tech information > installing body and fenders. Also,
http://modelaclub.com/archives/tech-tips/1211. For frame diagram: wescottsauto.com > tech information > 1935-40 frame diagram.
First step is to find and correct the root problem(s). This could include:
Door hinge screws loose.
Door hinge pins or hinge pin holes worn.
Structural deterioration of the cowl or door.
Structural deterioration or frame damage.
Deterioration or missing body to frame pads.
Improper shimming of body to frame.
Door hinges needing to be bent to set the door in opening.
Not an easy answer.