225/50R15 is the correct size tire when the 15x8 Campys are used on the front. Its the factory size for that application. DeTomaso had the habit of using a tire on the widest wheel it was designed for. This makes the sidewall stiffer, reduces slip angles, sharpens response to driver input. The 225/50R18 has a fairly wide tread, about 8", so it fits an 8" wide wheel better than you may think.
The tire will fit and work fine on a 7" wide wheel too if that's your preference.
Regardless what tire you select, keep the front tire OD in the range of 24" to 25"