40 is a bit too small for these fish. I wouldn't breed them in anything less than a 4ft 55 gallons and that may be tight depending on how big they get. Jacks typically top out at 10" but they can get 12". That being said I have heard that because of the genetic instability of EBJD BGJD tend to be smaller. You could try it for now but I would look for a larger home in the future.
I was thinking the same thing, that a 40 would be too small. I think I'll try them in the 40 and if they aren't happy I'll just move them into the 140 and hope for the best. They are in a 55 with 9 giant danios and are perfectly happy. The only issue is that my 140 is going in the space that my 55 is. I can't have both the 55 and the 140 set up.