I would have hated for you to start the process of trying to remove the shell from the truck frame and then find out about the nails. I see your's is an 86 as well so I would be shocked if nails weren't used in the construction.
I understand why Airstream used them, as I'm sure they makes things a whole lot more rigid, but I'm also sure Airstream never planned on people trying to take them apart
If the pontoons end up costing to much you could always use a couple of logs
Sounds like a neat idea and if you end up doing it you'll obviously have to post pictures as you make the conversion.