I read through this thread again and realized there are two issues to address.
First to be safe when setting door gaps during construction the frame should be dead level front to back and side to side. If it is not the frame flex could be on one end of the flex spectrum or the other.
Second after reading Patrick's advertisement on your car it appears to be a 4" round tube frame. I have never driven an original cobra but I have talked to a friend who has driven a couple, and was told the originals were not good at all. Some that use that design just up the thickness of the tube, others also add a lot more bracing to significantly improve the strength.
Take a look at this Kirkham frame and note the extra cross bracing to stiffen the frame. Hope this helps.
