[leegreen]

I'd prefer not to see the tank when standing behind or beside the vehicle, so tucked in behind bedsides / bumper. but that is just an opinion

Functionally it's more of a question for the area you want to drive your truck. You could measure tallest speedbump, steepest driveway by driving something with a similar wheelbase and rear over hang onto it then measuring different points.

5 1/2 sounds a little low for a vehicle with a long flat rear overhang, depends where you drive. Check your municipal definition of speed bump/hump height, but assume the guys shoveling the asphalt will pile it higher. Locally speed humps are supposed to be only 3" but in reality they are probably 6
Dips in the highway can bottom you out harder than bumps on local roads