My first post... Long time lurker.
Why do you say it's not a Highboy frame? Looks like a Highboy Crew frame to me.. Just missing a few things, and that loose cross member is from a 2wd. What does the VIN say, or has it been ground down? Should be in two places on the top passenger side frame rail. Once ahead of the front cab mount, once behind.
I'm curious too..
What's the height at it's tallest section?