Express Bed tailgate different than wide bed '31?

[Osage Orange]

I'd like to fabricate an express bed similar to an 89-A, since the originals are rare as hen's teeth and I am not restoring. What are the differences between an express bed tailgate and one for a 1931 A pickup? They look similar, but I'm guessing the dimensions are different, width and height. Still, I'd like to get as close to original as possible.