it would be safe to put on 215/40s but if your going to go with a 40 sidewall you might as well get 225/40s (i'm guessing from your 215 stated that you have 7.5" wide rims and 225s will fit as well).
if you want to fill in the extra gap, i'd bump it up to the stock sidewall (for gts) of 50 instead of 40. you're ride will be softer and your overall tire diameter will still be less than 1/2"