Start by studying these dimensions.
http://www.914world.com/specs/bodydims.phpYou car is an interesting puzzle. There will be some key things you will have to keep track of from start to finish.
Part of the problem is once the chassis started going south the repair person may have tried to bend things like the windshield frame to make the door fit less obvious. This makes some of the measurements you want to use unreliable. I will get you past that by starting with the lowest point on the car and working up. So there will be very few measurements you can count on as a baseline but you can get this back in spec.
I'm going to make some assumptions as a place to start for now. If the following turns out to be unreliable I can back up one step. I am going to assume at least the forward check sets are intact. (the donuts or pucks under the car) Post some photos and I will instruct you in another direction if they are damaged, rusted/repaired or missing. For now start here.
Here is where I would start. Set the car up on a level plain at the forward check sets. Set it level from right to left across the car.
Now bring the car up or down front or rear to get the left side longitudinal level. If you have achieved this on the left go over and check the right side, as well as the right to left at the rear check sets.
I'm not confident the rears will be intact but my goal is to determine if, when the welding was done the last guy caused the longitudinals right side to left side to be out of plain. So first we are looking for twist. Don't worry about anything else until you have addressed this.
Report back with this information and I will tell you where to go next.
I will need some more photos from you too.
If you can take a right side and left side photo of the whole car and photos of the check sets that would be helpful. Also photos of the longitudinal photos both sides.
This will take awhile to get through but if you have patients and a space to work where the car can sit without being moved I can help you get it straight.