I might be missing something but wouldn't you simply compress (something) to give you the tightness you desire? we don't know what if any on your car is repro with perhaps different tolerances than original. for example, restorers have stated that the repro pivot cups don't fit in as tightly as original. besides that, we all suspect the open cars have experienced a lot of twisting and stretching in their history. I don't know that original measurements always have an application today.
also I assume you would try to torque the stanchions somehow, off the car. I don't think you would want to try to fiddle with the cowl posts themselves, that would throw other fits out of whack, wouldn't it?