I bought a ladder rack from Amazon for under $100, and 2 pairs of J racks for about $40. We put 2 yaks flat in the middle, and 2 on the outside in the j-racks, and it fits great. Also it leaves your bed open for other stuff. We have a bike rack that sits on top of the bed rails, yet below the yaks. We can carry 4 yaks, 4 bikes, and still have room for coolers, and firewood underneath.
If y'all are paddling flat water (lakes), I would get at least a 10ft yak, if not 12ft. We borrowed a 8ft before we bought ours, and they will turn on a dime, but they don't like to go straight. My wife and kids have 10ft, and I have a 12ft, and you can really tell a difference how they track.