Slow but willing learner
One thing to consider is the relationship of the grill to the body. When I put the '36 together, I mounted the grill and hood. If the hood gaps at the grill and cowl aline, you know you are pretty good. I don't even know if you have a hood so this may not apply to you but I thought I would throw it out.At the front of one door, the door body line is completely above the one on the cowl. Looks like the cowl is at least an inch low. I have no way of knowing how high the tail panel should sit in relation to the cowl. Only info I had is it bolts directly to the frame and you'll screw up and have the body sitting way too high if you do anything else. I'll just take the support fixture out of the car and start from scratch. Yesterday I joined the hinge posts (with pillar off and quarter hanging) with a dimensionally exact crossmember because I found three degrees of variation in the angle of the posts. That would stand alone. I assumed what I needed to do was get door gaps between the quarter and door first but the reality is I need to align doors to cowl first, then skin that with a quarter regardless of butcher work required.
I do think you were right to start with the door gaps. That should tell you when the body is reasonably square.