First, check the hinges, and specifically, the hinge pins. The pins tend to wear after 40 plus years.
The door striker is not there to hold the position of the door. It's job is to keep the door closed, and maybe slightly adjust the depth of the door to the car, when the door is closed. The striker holds the door against the weather strip. It DOES NOT adjust the height of the door at the back of the door.
If you have bad hinge pins, you probably will have to take the door off the car. You might be able to remove the hinges one at a time, if you can support the door securely, but if you do that, you have to be careful. Good possibility of chipping paint on the front of the door, or back of the fender.
The hinge when closed, forms about a 90 degree bracket. Where the hinge bolts to the door pillar, you can adjust the door forward, back, up and down. This is for both the upper and lower hinge. Where the hinge bolts to the door, you can adjust the door in, out, up and down. This adjusts the front edge of the door. The front edge of the door has to be inside of the fender edge, or very slightly in deeper. If it is outside the fender edge, it can hit the fender edge, when the door is opened.
To adjust a door properly, remove the striker plate, temporarily.
You adjust the height of the front edge of the door directly. Try to keep the height of the hinges centered, do not have the hinge all the way up on the door pillar, and to compensate all the way down on the door. To adjust the height of the door at the rear, you manipulate the hinges forward and back. If the door is too low at the rear, you move the top hinge forward, or the lower hinge back, or both.
When you get the door fitting the door opening up, down, forward, and aft, then you adjust the door in and out to the weather strip. You check weather strip fit with a dollar bill. Open the door, put the dollar bill on the weather strip, hold the door closed by hand, and pull the dollar bill out. You want a definite drag, but if the dollar bill tears, the door is too tight in that area. Check the fit of the door at the bottom, and sides below the window. After that, check the fit around the window. Remember to move the top of the door in, or tighter to the weatherstrip, you can move the top hinge in, or the lower hinge out, but in any case. adjusting the fit better in one place on the door may or probably make the fit worse in another place. There is a lot of compromise in fitting a door to a car. If the fit is good on the lower part of the door, and you cannot get the top of the door to fit close to the weatherstrip, you can pull the top of the door frame, that holds the window, in slightly, and make it fit better. Not too far, the window still has to go up and down in the window run channel. Do the bending of the top of the door with the window down, and check how the window runs up and down after.
After you have the door fitting the opening good, and rests against the weatherstrip all around, then you can replace the door striker. Remember the door striker does not adjust the up and down of the rear of the door. The hinges adjust and hold that. The door striker adjusts only the depth of the door to the car.
If you have not adjusted door fit before, it will take a long time to do it. Make small changes, one at a time. Do one thing, check fit again, make a note of what you did, and what happened.