There is very little difference between a 39 and 40, so I don't see why you couldn't do that swap. Being the proud owner of a 1940, I only ask why would you want to do that swap?
The biggest complaint about straight axle trucks is how they handle, and how they stop. There are tricks to getting your stock front end to both handle and stop better. That being said... it's your truck and you could do whatever you want to it. Good Luck Mate!
