PVC pipe at 6" diameter by 120" long will displace about 120 pounds. If you use 96/6 = 16 of them, you will have plenty of potential flottion. That said, my suggestion is to forget the PVC plan. The pipe will be heavy, The raft will sit very low in the water, the hinged idea will work but it will complicate construction.
A much better plan is to build a mortar box style boat that will fit the back of the truck in the first place. If the wheel wells are 4 feet apart, you can build the boat, (call it a barge if you like) 47" wide. That'll give you better service than the pipe thingy. It will be much lighter, easier to build, and probably cost less. If you want to see a boat that has a pretty large following and is simple, cheap, stable, light, and proven....Check out the PD Racer. You can find it on the Duckworks web site. Plans are free. It is a sailboat but you are not obligated to use a sail. A small motor or a set of oars will make it go pretty good, but not fast. It will be faster and easier to propel than the pipe raft.