Frank,there are at least three different makes of these roofs but the idea behind their application is about the same. They include at least one design that goes over wood sheathing.They are all made so that they accomodate variations in car dimensions.
I believe you will find that if you start at the center of the car and work towards the ends that
the end pieces will accomodate any variation in length that you might have.
Try fitting some pieces together on the floor so
you can find out how much gap you should have.
If you can find a Car Builders Cyclopedia from
the late 20s or early 30s it should have drawings
showing how the roof fits together.
Let me know if you have a problem and I'll try to find someone who has the answers.
Jim