Well, I don't know why you'd need a spacer (I don't have an HHR), but you'd want to keep your current overall tire-wheel diameter the same front and back, surely? Thus, if you have 245/45-18's on the back but wanted a narrower tire on front, then going to 225 would mean you should INcrease the aspect ratio to maintain the same sidewall height. Thus you'd need 225/50's.
If you wanted the same diameter as your original 215/55-16's, then yes, you'd want the 225/40. But then the fronts will be a different diameter to the rears, so compared to that, the tire type is probably moot!