1.5" is the dimension of the aluminum cross bar. The shutter hinge and roof track provide 2.25" of lift above the roof mounting holes. It seemed like that would be enough per initial measurements.
I'll be the first to admit that I'm a wimp and it's been too cold to test fit the final assembly. I'll report when I do. I can alway add a spacer under the hinge if needed. There are larger shutter hinges available (I think the lift would be about 3" for those) but the metal thickness seemed overkill for this application.