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.
If you can get a hold of pdf of the original brochure it may have the correct spots to rest it so that it on the rack so it doesn't stress the deck. I have a Barnett butterfly (12ft) and it has diagrams and directions to top it.
Also invest in some ratcheting tie downs it will be the easiest and fastest way to secure it.
I routinely cartop a 17-foot folding sea kayak that weighs about 80 lbs. I use a standard rack system with a product from Thule called a "rollercoatser", which allows rolling the boat up onto the rack with minimal effort. I then use two ratcheting tie-downs on the rack and two more on the bow and stern.
At 150 lbs., I'd check what your roof rack is rated for. If you have an aftermarket system, you should be OK, but some factory racks are pretty cheesy.
I loaded the Dagger on my rack yesterday, by myself, and it went pretty smooth. I have a F250 with a Cargo Master rack. This is how I did it: stand the boat vertically on it's stern against the rear of the truck, rest the boat against the back bar of the rack, tip the boat onto the rack. It is not easy but doable. One you get the boat to the tipping point the weight of the boat above helps. My original plan of loading the boat upside down did not work as the beam was too wide, so deck up. A winch could probably be rigged to help loading and unloading. I'm working on that next.