This post from a serious Model A expert is the clearest possible presentation of the differences between '28-9 and '30-31 decklids, a question that comes up here continually. Note that the differences preclude bolt-on swapping, but are certainly within the range of fairly simple modification. Just some good info, done right by an expert: http://www.fordbarn.com/aforum/messages2/403115.html
Marco most certainly knows his stuff... the pics should be a big help to those who are unsure of their lid.
I've read Marco's posts over there before. There's not much he doesn't know about the Model A-probably more than Henry did. Thanks,Bruce.