Sorry, I forgot, but why are you downgrading from your 75 to a 40? No sump is ever too much. Bigger the better of course, but there's going to be a lot of extra room b/c I don't think you would have that big of a skimmer. Sure you'll have more water volume which makes keeping parameters in check easier, but if you want to get a smaller sump I would see no problem in that either. If you had a sump on your 75, where is it?
“Success is walking from failure to failure with no loss of enthusiasm.”
- Winston Churchill