I ran 18's with no dramas, it's all in the offset. I now run 18" Lexus mags which on full lock just catch the inner guard, but because the turning circle of a Super Cusrom is so good I live with it.
Can't recommend spacers as they put greater pressure on the studs. However saying that i had to fit longer High Tensile Steel studs to fit my original wheels. I think you may find that the Territory offset is not as suitable as the pre AU Falcon offset.
Regards
Rex