If it were me, I'd channel my inner Smokey Yunick and I'd cheat like crazy.
You could easily chop the top by half and inch and have nobody notice, angling the roof for better aero. And who would realize if the truck happened to be an inch and a half narrower than stock? I say nobody. Cheating is a lost art, and you need to help bring it back!
Seriously though, would a King Cab be eligible? I'd imagine that the longer roof would have slightly lower drag, along with putting some mass slightly farther back. Especially when you'll be chopping the top...
Good luck with it!.