I carry 4 full size bikes in my short bed truck upright. The only difference to your situation is that I have a hi rise truck canopy.
I put the first bike close as possible to the left or right wheel well and the front wheel turned parrellal to the front of the box. The other bikes fit closely to the first one in the same configuration. Pedals should be up and down so they do not touch the ajoining bikes wheel. Foam protection should be placed by the deraileaurs between 1st and second and third bikes. These can be any kind of old flat cushons from old lawn furniture etc. I use 3X2'X5' cushons placed between that protect all parts of the bikes as we sometimes shuttle bikes over rough roads. Even cardboard might work or foam water toys etc. Aligator Straps or ratchet straps are wrapped around the top tubes of the bikes in such a way that they will not slip and attached with hooks to the top of the bedrails or "O" rings connected to the bed walls. As there will be room on the outsde of the last bike, bins full of your stuff can be placed so wheels cannot move out. Make sure straps are wrapped around top bike bars so bikes cannot move or there is very little movement in case you are on rough roads.
Still room between rear wheels and the tail gate for more storage bins.
This is a very simple yet effective way of transporting up to 6 bikes in you box with out using up a lot of space. You can also use this method in reverse with bikes hanging off your tailgate, front wheels parrelal to the tailgate outside of the box, rear wheels sitting on the bed floor.
Good luck
Dave