Just speaking from my own experience, I wouldn't do it. Its much easier to find something with solid sheet metal and build from there then it is to fix rust. The frame on it might even be rotted and then your looking for that too. I would bet at the very least the body mounts are shot on the frame and will need repaired. Those old frames always rot out where to rubber mounts sandwich the steel. Last summer (09) I welded in floors, rockers, cab corners, and cab mounts in a 75 F250 High Boy. Awsome project and it came out great, but looking back at it there would have been less time and money in it just to find one solid from the get go or swap the cab all together.
It would be a fun project to take on, but I don't think it would be worth it in the long run.