Hello JKDChevy,
Over the years some 1931 Chevy car and truck radiators may have been repaired that changed how the radiator cap mounts to the filler neck. With that in mind, you might take a look at the radiator caps that are offered by: The Filling Station. Click on each cap to get full description and photos of mounting parts. This may give you an idea as to what might work for you. First thing to do is to make sure that your Eagle base will cover your radiators filler neck. From there, I think that it's up to your imagination as to what to do.
