Ummm....I really don't think your info is correct man. A 40 series tire is SHORTER than a 45, and so on. A 265 is wider than a 255...the 40 in the tire size is a percentage of the tread width that determines the height of the tire.
I would say that a 245/45 would be the biggest that I would go because a buddy of mine here in town has then on his Jimmy that's lowered 4-6 and he clear with no rubbing. I would say that the 2 of y'alls vehicles are pretty close to being the same height, so I wouldn't go much higher. Maybe a 255/45, nothing taller than that.