Generally you work from back to front when hanging and aligning sheetmetal. As mentioned, back in the day, production tolerances were such that gaps can vary quite a bit from car to car. Sometimes you have to make compromises that simply look good to the eye. Once the door is aligned to the rear quarter, you adjust the front fender - but that can be more involved than you want to get into.
Start by making sure the hinge pins aren't worn. Then, adjust the door up and down, fore and aft by loosening (just enough) the hinge to cowl bolts. Two of those bolts are accessed behind the kick panels. The hinge to door bolts allow adjustment in and out away from the body. Loosen one hinge at a time and make minor adjustments. A padded jack under the door and/or a helper can make it easier. The striker bolt may need readjustment or replacement too.
Judging by your picture, the door is definitely drooping at the rear. If the hinges aren't worn it may be as simple as loosening either the top or the bottom hinge on the cowl side, pulling the door up a little higher than level, and retightening the bolts. The striker will need to be move too.