You can make a wooden bed for pretty cheap--just make sure to use treated wood. You can make removable sides, and have sides that are different heights to make it more versatile. But I wouldn't sink too much money into it--almost every problem I have had with my cars can be traced back to rust/corrosion on bearings, axles, electrical wiring, brake lines, power steering lines, etc. I'm betting this truck has a lot of underbody rust that could cause a lot of problems down the road. Also, on a truck that old you have to make sure the thing is still safe to drive (solid frame and axles).
