The 1A Raingutter Tower is the only tower we offer a "spacer, or riser" kit for. All other tower the height is dependent on the surface being mounted to and cannot be adjusted.
Slight height differences, between caps and cabs of trucks is normal and typically should not pose an issue.
Still have questions? You can contact customer service here, or by calling 888-925-4621 M-F from 7 to 4 PT.