I am running a 255/70/15 on the rear(29" tall), & have 225/75/15's up front(28.25 " tall)......but plan to go with a 225/70/15 up front to get to about 27.5 on the fronts. I run rallys on my longhorn also (15x8's rear & 15x6.5's up front). You can figure the 275/60/15's to be right at 28" tall......the 285/70/15's im not sure on the height, but think they may be really tall compaired to what you are planning to run up front. best of luck,
L