Sounds to me like you need a rack for the tailgate end of your truck as well.
Is this how you are going to transport it all the time? In/on your truck?
If so, then why not do it right and build a proper rack for the front and back?
Another option is to build a hitch on the front of your truck that you could put a T shaped bracket in the receiver. It would come up slightly higher than your cab. It would be removable.
So in this instance your boat would ride above the cab and towards the front.
(You could do the same for the tailgate end as well)
Where do you live? You need a good fabricator- like me
To do it right and with metal, you are looking at a few dollars.
If you are going to trailer it once you get it home, then you can cheap out with some lumber, screws and crazy glue
If anything, you now have a little more to think about.
Carry on!