For anyone interested I finished up my project to haul our kayaks on our class B. It's a challenge as the van (a Promaster) is so high plus has air conditioner, solar panels and air vents to avoid. I had to design/build my own rails and crossbars to be as low as possible. I used Thule Hullivators for the lift mechanism. I can load the kayaks into the cradles and strap them down before lifting into position. The 28" step stool helps.
The Hullavators have gas springs that assist up to 40#. Our kayaks are 39-40# so are nearly weightless on this setup. The cradles are 67" apart and, since they work independent of each other, it's a bit awkward on the lift. (needs some practice).
Road test: At 65 mph there was no noticeable wind noise. And I did not have the cockpit covers on for the test.
Height: The highest point of cockpit rim is 10'-1" high. (The air con was at 9'-1-1/2"). Yes...I have to watch the tree limbs.
Some photos... --KenA