I've been searching for a while but I've found a decent full-sized pickup for the USFS patrol/utility for the 1980s. Not an actual pickup bed like the 90s unit but as far as I've seen most of the older pickups had rear bodys like this.
If you have any plans on adding some more USFS rigs into the mod I may be able to help with reference. I found a group while back that has a ton of photos of the old Chevy and GMC engines along with some others and even the Unimogs that the USFS had.
(Update:)
I found this photo which was interesting enough that I figured I'd tack it onto the post. Doesn't show anything new that isn't in my other post on this private EMS company.