how t set door on 1970 cutlass convertable

Pretty much trial and error. Realize you do not want to scratch paint, thats what makes it a pain to do. Have two people to do this, and get the door up there and bolted in place just tight enough to hold it. From that point it is doing whatever it takes to make a nice matching body seam. Put towels and such over painted areas. Carefully close the door to check for alignment. Do not be slamming it shut & don't be forcing it shut.

Or are you just trying to adjust it? If that is the case then it sounds like the door hinge pins need replacing. It is a big heavy door.

A door jig will make the job a lot easier. It might be better to have a body shop do the job for you.