Greetings & Welcome!
Nice van! Please don't listen to any of the fools that will tell you to gut it out and start over... They are getting paid big money to convince others to make horrible choices. By leaving the floor, walls, and ceiling intact, you can save some big bucks, and the chances of actually improving on them are practically zero.
Measure your total height, and always give yourself at least 6" of leeway for any low parking garages, drive thru's, etc. Always better to be safe than sorry.
We'll look forward to watching your conversion. Give a shout if we can help!