What do you mean by 'convert a truck'?
You don't need to be mechanic; you just need to use common sense and we're here to help you with where to look/interpret the pictures you supply us.
As
@JoshT indicates, the fact that distance between cab and box doesn't appear to be constant is red flag to us. As is the damage to the back of the fenders.
It's either bent box or bent frame.
And we need to see what is 'under the skin':
If the channels don't have nice straight sides/sit flat on the frame, then you have box issues
If you put straight edge (doesn't matter what it is, just so long as it is straight) on the frame; does it sit flat.
We need those couple additional pictures.
What part of California are you from? It's pretty big state
(My company has plants in both LA and SF areas, that I can talk to locals about recommendations).