[LongBox]

Not sure if you can do this where the truck is now, but I am thinking pressure washing would be a good start. When I got my 69 home, I pressure washed the entire underside, and then, carefully did under the hood wherever I thought I could without causing damage.

After the pressure washing, a degreaser, then scrubbing. The suggestion of removing and painting the inner fenders and rad support are really good ideas.

This is so worth it, as after you do this, it becomes a pleasure to work on the truck.