I think a liberal application of WaxOyl will solve most of the surface rust issue and also make sure it goes inside the chassis legs through the holes as corrosion usually takes hold from inside
Once applied (which is easy but messy) then you can see what else is left to tackle but you would be amazed how it will transform the majority of the truck
Be sure to use eye and hand protection and not to smoke as it is flammable and can cause damage to eyes and skin