Vic,
I have not tried the 12 Caribou on for size yet (bedliner & dually wings to come this week). I will hopefully get to test fit by the end of the week. I did however, measure the cab height, and 42. 5" came about 2/3 up the 3rd brake light. It looks like approx. 45" is dimension to clear. I'll let you know when I try it on.