325/65/18 = 325mm wide (12.8in) / 65% sidewall of width = 34.6in tall tire
325/60/18 = 12.8 in wide / 33.4 in tall tire
So the difference is about 1.2" in total height of the tire
Additionally, the first will make your truck about .6" taller.
On an aftermarket wheel and an AS, I dont think you can fit the 325/65/18.