I would think trading would make more sense than paying somebody else professional rates to chop the roof off and add a high top. And I don't know if the van conversion companies would touch an older van or not. They like to work with new ones, with no rusted bolts or dimpled skins to work around. But if you are in a metropolitan area, I'll bet there is a customizing shop somewhere that would give you a quote on the job. Check the Yellow Pages under Custom Vans and similar wording.
Lot s of hits on Google for custom vans, so maybe an evening spent emailing them would get some responses as to whether they do that sort of thing.