['72 longhorn dually]

not sure where to put this. I have a '72 Chevy c30 i am rebuilding, in the process of getting it to body work. my question is how does one clean inside the outer and inner bed sides? ( not the inside of the bed (that's wood) the actual sides them selfs.)what product should i use for rust prevention/ coating? is there a cost friendly 360 degree spray system to use? ( it would need to be approx. 3' long) The only solution i can come up with is get a wire wheel on a 24-36" extension and go to town, thoughts? also i know i can drill out the spot welds and split them apart, not something i have the room and want to do. any and all suggestions are appreciated.