73 up K frame

Agreed, hot tank it first at some local machine shop, then weld all the seams then if you can,get it sand blasted. If not then after welding just wire brush it and soak it with epoxy then paint or just rustoleum.

Eastwood makes "internal frame coating" and what I would do after final blasting or wire brushing, is spray a can of this stuff inside the K frame, then paint the outside.

http://www.eastwood.com/internal-frame-coating-w-spray-nozzle.html

Should last you a hundred years then.