Pull the inner door panel - arm rest, pax. grab handle (for right side - unclip the plastichrome end covers to access the screws), door latch handle escutcheon and stainless metal handle surround, window crank, and door lock button. Pry the door panel from the door, starting at the bottom. Lift the panel upward to unhook it from the upper edge of the door. You should be able to get at the door innards now. (I'm assuming any vapor barrier in there is long gone by now. Remove as required, if it's still there.)
There's about a 1/8 inch dia. rod with a threaded end, and a Nylon adjuster that bears against the lock mechanism, toward the back edge of the door. (This is from the handle to the lock/latch mechanism - there's another rod in there too, but it doesn't seem to go out of kilter as much as the longer unit.) Theoretically, there's a metal clip or keeper, to hold the Nylon adjuster where it should be. It may or may not be there. If it is, the lock mechanism can still get out of adjustment, because the lock rod has a mild "Z" bend (what other shape would it be for such a car?) in it, and the bend can become exaggerated with time and usage, in effect shortening the rod and making it more difficult to lock the car. Rebend the rod as required; I'd suggest taking as much of the bend out as possible, and still being able to reinstall the threaded end of the rod back into the latch push plate up inside the door. Adjust the Nylon adjuster as required. You can test this by latching/locking and unlatching/unlocking the door with it open. Just remember you have to flip the latch to the latched position on the door jamb, as though it engaged the striker upon closing the door. If you do this, the door will "think" it's latched, so you can activate the lock mechanism for adjustment. Yes, one can reach all this stuff inside the door without further disassembly, but it ain't easy. Plan on some skinned knuckles from the sheet metal edges in the door.
Now is a good time to make sure your window seats properly in the upper door frame, too. If not, loosen the large screws in the window regulator mechanism, and roll the window all the way up. It should be aligned with a uniform gap all the way around the upper frame, when it gets close, and seat evenly at the end of the travel. If not, jiggle it around a bit while maintaining pressure on the crank - it should shift the regulator around as needed. Keep the window rolled up tightly to make sure it's seated where it should be, and tighten the regulator attachment screws.
Also while you're at it, make sure the drain holes in the bottom edge of the door are open, and check it out for rust. Use some anti-rust of some sort (tar, whatever - just make sure the drain holes stay open) as needed.
Now you can put it all back together, reversing the order described above. It should be all squared away. BTW, if I missed something RE: Your original question (like, maybe you need to replace the lock cylinder) this is how you get to it also. There's a metal clip inside the door skin that holds the lock cylinder in place. Again, all this stuff is a real bear to get at, but it can be had.
--Jack M. (been there, done that) & 71 ZCAR--