Very FIRST thing .... figure out how you want the bed, lengthwise or crosswise. Everything else vectors off of that decision. Depends a lot on how tall you are and how you sleep.
For a small van, you only have one choice, lengthwise.
In my GMC Savana van, the distance across the back just over the wheel wells is 74"-76", so I have a foam mattress, twin size 38"x74", crosswise in the back. Being foam, it conforms to the curvature of the walls. However, and this is a very big deal, since the mattress fully takes up the entire cross width, I cannot put regular insulation and paneling in there like most people seem to want to do.
For Ford or Chrysler vans, it'll be somewhat different considerations. Everything is a compromise.