On the cars the buckets are similar if not identical but the stands are different. In '37 they were a separate item while in '38 they were part of the core support. I don't really know if that applies to trucks though. Someone who can give you a definitive answer will be along before long.
For what it's worth
this site sells fiberglass reproduction parts and shows that 36-40 were two piece similar to the '37 car. Scroll down almost to the bottom. If those are faithful reproductions it seems that the '37 and '38 would be the same..
