The rear bow in some convertibles pivots as the top is raised or lowered so it will fold into the top well. A good source of information regarding convertible tops is LeBaron Bonney.
1-800=221-5408.
The book;The V8 album shows the measurement as 38 inches from the rear seat top cushion about 4 inches ahead of the back rest to the rear top bow .try that it will put you in the ball park.John