Get up there and put some weight down on the areas your considering. Then look between the headliner and the roof with a light to verify. You have to get in there to run the cables anyways so should get you some viewing space.
We do many of these trucks for counties each year without ever pulling a headliner but Ive never measured...
If theres a brace you should be able to feel it. You can usually pull the dome light for some viewing area also...
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