Helps to have the platform flat (build a leveling platform in the rear / factory sliding cargo tray)
Can go diagonal to fit 5'10" without any modifications.
If you took out the rear seat bottoms (easy), you get a lot more north-south room. May need to fill the void.
Lots of other platform options out there if you want to add something more dedicated.
I built this for my brother's 4th Gen...
DIY SUV Bed Platform (5th Gen 4Runner & Others) - YouTube