(Disclaimer: I'm no expert, but I've been reading a lot.
You'll get different flavors from 60 and 20 than 40... if that's all I had, I'd probably sub something like a couple ounces of crystal 60 and 6 ounces of crystal 20. It won't be the same as 8 ounces of crystal 40, but it's not an unreasonable substitution, IMO.
What I've learned so far, though, is that it's not just a matter of the numbers; the darker crystal malt imparts different flavors altogether (raisin, plum, etc.) than the medium and light ones.