WAIT...... nono DON'T use the expanding foam insulation used for home construction. It WILL absorb moisture and WILL rot your sheet metal. Go to an automotive supply store and you can get expandable spray foam used to fill unibody sides for sound deadening. There are many kinds of automotive foam, make sure you get the right kind for what you want. oh yeh, it is kind of pricey too, more than the Home Depot kind.

1957 Chevrolet 1/2-ton Stepside LB in the Gallery
My Photobucket shots

The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." - John Stuart Mill (1806 - 1873)