Is it a one piece or three piece neck? To my knowledge all of the F40s came with a three piece neck with the center stripe, early 50’s thru the early 80’s. Some had mahogany/maple/mahogany, others maple/mahogany/maple. My two are the latter. The F40 headstock face plates had the chesterfield logos, fingerboards had square block MOP inlays. I don’t believe the D35 ever had a three piece neck, chesterfield faceplates or inlays. Also I think in the early to mid 70s was the F40 body changed from a narrower OM kind of body to a deeper mini jumbo Shape.
I’m not aware Guild ever using a Spanish heel construction, maybe on early Mark series classical nylons, but not that I’m aware of. It would be obvious looking inside. It either has a traditional neck square block, or a U shaped tongue where a neck block would normally be. Of course it is possible someone has altered the guitar, we come across that here quite regularly.