We use Hullavators for our kayaks and appreciate the mechanical advantage they give us. However, ours are designed to mount either to the left or right side of the roof- the "drop to the side so you can more easily load them" feature precludes a center mount. I'd also check the weight limits on the Hullivator v. the weight of the wherry hull. Good luck, and have fun.

   The Hulivator lifts 40 pounds.  You have to squeeze two handles to release latches and raise the boat......lifting the remaining 50 pounds of boat and Hullivator.  Our 17ft boats rest on three bars, and since I don't like a lot of boat over the hood we have only 4 ft +/- foward of the front rack, old Yakima, , and 4 ft +/- that hangs out past the rear bar at the tailgate.  Either way the space between the Hullivators is 6 ft.  I have good span but not that good. Since weight loading on my boat is not symetrical I can sometimes "bump"/ release the rear and lift at the center and load it solo.  I rarely solo off the truck.  I find the three racks preferable to only two for the following, 

1.The sea kayakks in the Hullivators, at 24" and 23" beam fit loose in the carriers . Carriers made for larger boats, wider. Thus the boats tend to wander in the rack. The third rack with cradles and strap "lock" the boats in place.  Bow and stern lines not required. 

2. We often end up on rough roads, and/or miles of roads RV traveling and I don't like long cantileavers of boat hull sticking out past the racks.  I do not want to block my view like I was driving a car carrier truck. 

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The mounting on the Hulivator  , as mentioned by others, requires the carrier ends (pivot point)   to be outboard of the roof of the truck such that the carriers hang straight down.  Wife's 62# kayak isn't far enough out, needs adjusting, such that the , steel of the carrier hits the side of the truck.  Center line carry won't work. The boat in place appears to be mounted above the side of the truck. 

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With a 100 pound boat, it will be with gear, you will bang the side of the truck loading solo. 

   I would look at roller gear mounted to the rear of the shell so you can push the boat  up over the rear of the truck onto the racks.  Bolt the rollers to the bars to prevent movement of the mounted roller. When I had that set up the clamps rotated on the round bars once and the boat got scratched. 

   I have no experience with the Expedition Wherry, but we sometimes car top our Shearwater Double on a 4WD Sierra.  For comparison, the SWD is about the same length as the EW, but 10” narrower and at least 15# lighter.  I originally looked at the Hullavator but the SWD is over the spec weight limit so I am sure that the EW is also.  There are other systems but I don’t know much about them.  I initially used foam blocks on both rails but switched to a set of rollers on the back and saddles on the front.

For two of us, loading the SWD takes some effort, but it is doable.  My bride carries the stern while I lift the bow up onto the roller, then both of us push the boat forward.  I can and have loaded the boat solo, but it is a PITA.  When loading solo, I use a C-Tug cart to position the boat with the stern directly behind the truck and the bow off to the side and forward enough so that it overlaps the aft rail.  After placing a heavy towel under the stern, I lift the bow up over my head and climb a step stool until I can set the bow on a roller on the aft rail.  I then reposition the step stool, pick up the stern and lift it forward climbing the stool until the bow tips over resting on the forward rail.  By standing on the tailgate, I then position the boat as desired before securing it.   I am small but fit and find loading the boat solo somewhat risky.  It would be easy to slip while holding +50# over my head and climbing a stool.  For me, loading single handed is an absolute last resort.  Looking at the heavier, wider EW, I believe that it could be loaded single handed via my method, but it would be difficult unless you are considerably bigger/stronger than I am.

Over the years, I have been carrying my boats further back than in this picture.  The boats appear to be more secure (less buffeting), the truck gets better mileage and there is less wind noise in the cabin.  I now position the bow somewhere at or behind the windshield depending upon the boat.  The large overhang off the back won’t really be a problem because you can temporarily reposition the boat forward for the ferry ride then move to a better position for highway speeds.

One other consideration is water accumulation in the cockpit during rain.  If you find it better to car top the boat right side up, you might want to consider a cockpit cover like used on kayaks.  Another approach is to add some type of bailer like an Anderson or a venturi to the boat.  That would be handy when rowing in chop in addition to draining the boat during transport.

   I appreciate the very helpful advice. I think this is the first time I've ever posted on a forum. Thank you! It sounds like maybe three rows. Roller in the back, harness in the middle and bar up front. Sounds like with the extra weight the hullavator would be banging the side of the truck.  I think I can do it. I have a wife and 11 and 4 yo boys but will need to do it solo at times for sure and at least once in Alaska. Really enjoying the building process so far. Thanks again.  Meher

   Also if anyone would like to suggest the exact roller, harness and bar for the three rows it would be welcome. I assume the front could be any bar maybe with the foam covering clc sells. 
thanks again

   Any updates to best way to cartop the EW?

This isn't a suggestion on how to cartop it, but, being an owner of an EW, it's something I'd never do.  The boat's just too long and too heavy, imho.  I've always trailered it.  Lot less hassle, and you can even back the boat down the ramp and effortlessly launch the EW off the partially submerged boat trailer. Same with pulling out.  Just a suggestion.