I think I got the bow worked out.
With this particular kit (again, an older issue), the rear wall of the cab was cracked. The bed sides have little channels that line up with slots between the back of the cab and the rear wall of the cab. With the rear wall significantly cracked, this was pulling the bed downward. I glued the damaged rear wall back together, shimmed with some strip plastic, and here's where we are. Still needs wheels attached, some details remain.