The interior of the Transit can be deceiving, not only are the walls concave or taper in at the top, the ceiling is also concave AND tapers in height from front to back. And the height from the floor to the windows (or window cutouts) changes front to back.
For someone with OCD that
requires things be plumb and square, it could lead to madness. When dealing with organic shapes, it's probably best design-wise to go with the flow and adapt/incorporate these curves. It's more
work, though. Everyone knows how to layout and make 90° cuts, and tools are made with this in mind, cuz home construction is dumbed down so anyone can do it (that's also why they use wood, even though it's not the best material for the purpose, because it's primitive and simple).
Like Michael said above, scribing some wall shims to make the wall, or parts of it, perpendicular to the floor is one solution, but it does create wasted space as you add several inches to the wall at floor height in order to be flush at the top. There are several methods to scribe, here's one that works but takes a few steps to get the final curve:
I’m making a wooden toolbox for my pickup bed. How do I make a template to fit over the curved wheel well?
www.woodmagazine.com
Here's a how-to using a scribing tool for van walls. Same method as above, but bigger and a video
Here's a convoluted description of using a tick stick. I used a version of this method to make snug cabinets for my old Sprinter.
When I did Marble and Granite work, we used wood strips and hotglue to make templates. It's a little wasteful for materials, but will give you an exact full scale template. You make curves by using several pieces then matching the overall curve. Industrial hotglue dries in seconds, so you only need to hold the wood strip in place for a few seconds before going to the next.