Ran into this issue when I tried to assemble the top bows on my coupelet.
Here's the facts, The porthole wood is new and correct (verified from original pieces. Rear Top bow is original, cleaned and painted. Side porthole brackets are original and untouched.
The problem is the rear bow does not match the side brackets. The body wood frame is 51" inches wide where the porthole brackets attach. As assembled on the garage floor the porthole brackets measure 55" across. Which means I have to bend the ends of the brackets to get to 51" wide.
My theory is over time the rear top bow has expanded and now doesn't fit. The car sat for forty years in a very humid environment, now it is in a very dry environment.
Here photo's of the issue:
I have soaked top bows in water that have set for many years for a couple days then use a string or wire and pull them back into place. Works for me but do not be in a hurry to move them or they will crack, you have to remember most of these things where straight at one time then steam bent.
Amazingly we assembled one side on the car and then slowly moved the other side into position and it fit, no cracking or funny noses. Soaking in water with a slight pull was the second choice.
On the car the rear bow is level and swings up and down easily. I am concerned about the joint, will need to beef it up somehow. I can't see two little wood screws on each side lasting forever.