There are no accurate measurements for your '28 Convertible Sport Cabriolet that I am aware. There are VCCA members that own twins to your car so might be able to take measurements of their cars. Even Coupes might have some parts the same (like main sills, front door posts, etc.) Most of us have used what ever wood remains to use as patterns. Fabricate the wood parts to fit the sheet metal (if in reasonably good shape). I have used relatively inexpensive pine or scrap lumber to make patterns or sub-assemblies. One door post required 5 iterations to get it to properly fit as the remaining pieces had enough deterioration that some angles that were assumed to be 90 deg. were in fact about 7 degrees off from 90. If you take on the project with the attitude that a darn pile of metal will not defeat you it will work out.

The Fisher Body manuals help to understand the types of shapes, joints, metal parts that are common for that period. They also show techniques to repair parts that rotted, broken or missing. I have used several types of repair joinery (lap, offset, plated, etc.) to save a wood part for use or as a pattern. It can be a bunch of fun and learning if you have the right mind set.