Used to cartop a small boat all the time, upside down always worked best for me regarding mileage, ease of loading etc.
My routine was to get the boat to the truck (explorer) stern to and upright, then stand it up leaning against the truck, which put almost half the length above the truck, so it was a simple effort to lift and slide it the rest of the way on.
I made up a roller bracket that sat in the space between hatch and roof and allowed me to slide the boat up without effort. Made from a pair of rollers from an old trailer I tore up, bolted to a length of aluminum "C" nothing held it to the truck except weight and the bend.
With a bigger boat (mine was 11ft) you'll have more weight in the air so the lifting would actually be less, but unless you can get under the boat to pull it on all the way, or use a winch, I'd rig a clamp on wheel that fits the bow and rides in a three sided wood track in the center. My truck was a beater so I didn't mind having to climb on the roof and pull it on the rest of the way.